RFR: 8347958: Minor compiler cleanups relating to MandatoryWarningHandler [v2]

[Archie Cobbs]

> After recent improvements to Lint-related classes, there are now a few minor cleanup opportunities relating to `MandatoryWarningHandler`.
> 
> 1. The new `LintWarning` class provides access to the associated `LintCategory`. This means that the `LintCategory` parameter being passed to the `MandatoryWarningHandler` constructor is no longer needed.
> 1. The `prefix` string being passed to the `MandatoryWarningHandler` constructor can (in most cases) be inferred as well.
> 1. The field `Check.sunApiHandler` (which has type `MandatoryWarningHandler`) is no longer used and can be removed.

Archie Cobbs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'master' into JDK-8347958 to fix conflict.
 - Some cleanups relating to MandatoryWarningHandler.

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Changes: https://git.openjdk.org/jdk/pull/23167/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23167&range=01
  Stats: 61 lines in 3 files changed: 28 ins; 22 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/23167.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23167/head:pull/23167

PR: https://git.openjdk.org/jdk/pull/23167