RFR: Fix simple warnings [v2]

[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.

Nir Lisker has updated the pull request incrementally with one additional commit since the last revision:

  Add newline

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/229/files
  - new: https://git.openjdk.org/jextract/pull/229/files/941c4381..cc54e8b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=229&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=229&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 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