RFR: Remove dead code

[Jorn Vernee]

Remove the last little bits of dead code. As found through code coverage report.

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Commit messages:
 - remove some more dead code
 - remove some dead code from ClassSourceBuilder

Changes: https://git.openjdk.org/jextract/pull/158/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=158&range=00
  Stats: 47 lines in 3 files changed: 1 ins; 36 del; 10 mod
  Patch: https://git.openjdk.org/jextract/pull/158.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/158/head:pull/158

PR: https://git.openjdk.org/jextract/pull/158