Remove detection of rustup and cargo in 'missing extern crate' diagnostics

Previously, this would change the test output when RUSTUP_HOME was set:

```
---- [ui] ui/issues/issue-49851/compiler-builtins-error.rs stdout ----
diff of stderr:

1       error[E0463]: can't find crate for `core`
2          |
3          = note: the `thumbv7em-none-eabihf` target may not be installed
+          = help: consider downloading the target with `rustup target add thumbv7em-none-eabihf`
4
5       error: aborting due to previous error
6
```

Originally, I fixed it by explicitly unsetting RUSTUP_HOME in
compiletest. Then I realized that almost no one has RUSTUP_HOME set,
since rustup doesn't set it itself; although it does set RUST_RECURSION_COUNT
whenever it launches a proxy. Then it was pointed out that this runtime
check doesn't really make sense and it's fine to make it unconditional.

0 files changed, 0 insertions, 0 deletions