RFR: Fix simple warnings

[Nir Lisker]

Fixes for warnings:

* Missing `@Override` annotations
* Unnecessary code (casts and semicolons)
* Unused imports

There are more complicated warnings that should be looked at separately like resource leaks, potential null access, and unused variables.

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Commit messages:
 - Fix whitespaces
 - Remove unnecessary code
 - Update copyright year
 - Add missing @Override annotations
 - Remove unused imports

Changes: https://git.openjdk.org/jextract/pull/229/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=229&range=00
  Stats: 41 lines in 17 files changed: 7 ins; 13 del; 21 mod
  Patch: https://git.openjdk.org/jextract/pull/229.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/229/head:pull/229

PR: https://git.openjdk.org/jextract/pull/229