RFR: 8347958: Minor compiler cleanups relating to MandatoryWarningHandler

[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.

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Commit messages:
 - Some cleanups relating to MandatoryWarningHandler.

Changes: https://git.openjdk.org/jdk/pull/23167/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23167&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347958
  Stats: 63 lines in 3 files changed: 28 ins; 23 del; 12 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