RFR: 8279528: Unused TypeEnter.diag after JDK-8205187

[Aleksey Shipilev]

SonarCloud reports that `TypeEnter.diag` is not used. The last use was removed by [JDK-8205187](https://bugs.openjdk.java.net/browse/JDK-8205187). We can probably save a few bytes/cycles by not retaining it.

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Commit messages:
 - 8279528: Unused TypeEnter.diag after JDK-8205187

Changes: https://git.openjdk.java.net/jdk/pull/6971/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6971&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279528
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6971.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6971/head:pull/6971

PR: https://git.openjdk.java.net/jdk/pull/6971