Plugins deprecation: don’t suggest simply removing the attribute

Building Servo with a recent Nightly produces:

```rust
warning: use of deprecated attribute `plugin`: compiler plugins are deprecated. See https://github.com/rust-lang/rust/issues/29597
  --> components/script/lib.rs:14:1
   |
14 | #![plugin(script_plugins)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
   |
   = note: `#[warn(deprecated)]` on by default
```

First, linking to https://github.com/rust-lang/rust/issues/29597 is not ideal
since there is pretty much no discussion there of the deprecation
and what can be used instead.
This PR changes the link to the deprecation PR which does have more discussion.

Second, the “remove this attribute” suggestion is rather unhelpful.
Just because a feature is deprecated doesn’t mean that simply removing its use
without a replacement is acceptable.

In the case of custom lint, there is no replacement available.
Prefixing a message with “help:” when telling users that they’re screwed
honestly feels disrespectful.

This PR also changes the message to be more factual.

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