Don't include current rustc version string in feature removed help

The version string is difficult to properly normalize out, and removing
it isn't a huge deal (the user can query version info easily through
`rustc --version` or `cargo --version`).

The normalization options were all non-ideal:

- Per-test version string normalization is nasty to maintain, and we
  need to maintain `n` copies of it.
- Centralized compiletest normalization (with a directive opt-out) is
  also not ideal, because `cfg(version(..))` tests can't have those
  accidentally normalized out (and you'd have to remember to opt-out).

compiler/rustc_expand/src/errors.rs

@@ -162,7 +162,6 @@ pub(crate) struct FeatureRemoved<'a> {
     #[subdiagnostic]
     pub reason: Option<FeatureRemovedReason<'a>>,
     pub removed_rustc_version: &'a str,
-    pub current_rustc_version: &'a str,
     pub pull_note: String,
 }