RFR: 8347958: Minor compiler cleanups relating to MandatoryWarningHandler [v3]

[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 incrementally with one additional commit since the last revision:

  Restore the explicit LintCategory constructor parameter.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23167/files
  - new: https://git.openjdk.org/jdk/pull/23167/files/0ce44d63..87e44347

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23167&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23167&range=01-02

  Stats: 26 lines in 3 files changed: 12 ins; 3 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