RFR: 8338834: Remove unused import declarations in java.compiler

[Pavel Rappo]

Please review this simple cleanup. I tested it first by usual means, and then additionally compared documentation before and after the change.

How did I end up with this cleanup? I finally got tired of an eyesore which greyed-out unused imports in my IDE are. So I removed them as per IDE suggestion and then had some fun chasing bugs. Who would've thought that removing imports can be so painful?

During my bug chase, I created the following issues, from which you could guess the kind of bugs I ran into:

  * https://youtrack.jetbrains.com/issue/IDEA-358082/Import-of-a-symbol-referred-to-by-snippet-target-is-considered-unused
  * https://youtrack.jetbrains.com/issue/IDEA-358073/Support-changes-in-documentation-inheritance
  * https://bugs.openjdk.org/browse/JDK-8338833

And a bonus one:

  * https://bugs.openjdk.org/browse/JDK-8338838

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/20679/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20679&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338834
  Stats: 56 lines in 29 files changed: 0 ins; 46 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/20679.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20679/head:pull/20679

PR: https://git.openjdk.org/jdk/pull/20679