Remove detail from label/note that is already available in other note

Remove the "which is required by `{root_obligation}`" post-script in
"the trait `X` is not implemented for `Y`" explanation in E0277. This
information is already conveyed in the notes explaining requirements,
making it redundant while making the text (particularly in labels)
harder to read.

```
error[E0277]: the trait bound `NotCopy: Copy` is not satisfied
  --> $DIR/wf-static-type.rs:10:13
   |
LL | static FOO: IsCopy<Option<NotCopy>> = IsCopy { t: None };
   |             ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `NotCopy`
   |
   = note: required for `Option<NotCopy>` to implement `Copy`
note: required by a bound in `IsCopy`
  --> $DIR/wf-static-type.rs:7:17
   |
LL | struct IsCopy<T:Copy> { t: T }
   |                 ^^^^ required by this bound in `IsCopy`
```
vs the prior

```
error[E0277]: the trait bound `NotCopy: Copy` is not satisfied
  --> $DIR/wf-static-type.rs:10:13
   |
LL | static FOO: IsCopy<Option<NotCopy>> = IsCopy { t: None };
   |             ^^^^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `NotCopy`, which is required by `Option<NotCopy>: Copy`
   |
   = note: required for `Option<NotCopy>` to implement `Copy`
note: required by a bound in `IsCopy`
  --> $DIR/wf-static-type.rs:7:17
   |
LL | struct IsCopy<T:Copy> { t: T }
   |                 ^^^^ required by this bound in `IsCopy`
```

3 files changed, 3 insertions, 3 deletions

diff --git a/tests/ui/indexing/index-help.stderr b/tests/ui/indexing/index-help.stderr
index 1291bf2a461..4ec28ddf871 100644
--- a/tests/ui/indexing/index-help.stderr
+++ b/tests/ui/indexing/index-help.stderr
@@ -4,7 +4,7 @@ error[E0277]: the type `[{integer}]` cannot be indexed by `i32`
 LL |     x[0i32];
    |       ^^^^ slice indices are of type `usize` or ranges of `usize`
    |
-   = help: the trait `SliceIndex<[{integer}]>` is not implemented for `i32`, which is required by `Vec<{integer}>: Index<_>`
+   = help: the trait `SliceIndex<[{integer}]>` is not implemented for `i32`
    = help: the trait `SliceIndex<[{integer}]>` is implemented for `usize`
    = help: for that trait implementation, expected `usize`, found `i32`
    = note: required for `Vec<{integer}>` to implement `Index<i32>`
diff --git a/tests/ui/indexing/indexing-requires-a-uint.stderr b/tests/ui/indexing/indexing-requires-a-uint.stderr
index 38e7881dcc6..3041c2c99a1 100644
--- a/tests/ui/indexing/indexing-requires-a-uint.stderr
+++ b/tests/ui/indexing/indexing-requires-a-uint.stderr
@@ -4,7 +4,7 @@ error[E0277]: the type `[{integer}]` cannot be indexed by `u8`
 LL |     [0][0u8];
    |         ^^^ slice indices are of type `usize` or ranges of `usize`
    |
-   = help: the trait `SliceIndex<[{integer}]>` is not implemented for `u8`, which is required by `[{integer}; 1]: Index<_>`
+   = help: the trait `SliceIndex<[{integer}]>` is not implemented for `u8`
    = help: the trait `SliceIndex<[{integer}]>` is implemented for `usize`
    = help: for that trait implementation, expected `usize`, found `u8`
    = note: required for `[{integer}]` to implement `Index<u8>`
diff --git a/tests/ui/indexing/point-at-index-for-obligation-failure.stderr b/tests/ui/indexing/point-at-index-for-obligation-failure.stderr
index df4d7cc0683..4cced22789f 100644
--- a/tests/ui/indexing/point-at-index-for-obligation-failure.stderr
+++ b/tests/ui/indexing/point-at-index-for-obligation-failure.stderr
@@ -2,7 +2,7 @@ error[E0277]: the trait bound `String: Borrow<&str>` is not satisfied
   --> $DIR/point-at-index-for-obligation-failure.rs:5:9
    |
 LL |         &s
-   |         ^^ the trait `Borrow<&str>` is not implemented for `String`, which is required by `HashMap<String, String>: Index<&_>`
+   |         ^^ the trait `Borrow<&str>` is not implemented for `String`
    |
    = help: the trait `Borrow<str>` is implemented for `String`
    = help: for that trait implementation, expected `str`, found `&str`