[foreign-abi] RFR: Miscellaneous cleanups

[Jorn Vernee]

Hi,

This patch applies some general cleanups suggested by IntelliJ, such as:
- Removing unused imports
- Fixing typos
- Applying `final` when possible
- Using Optional::orElseThrow instead of Optional::get where it's uncertain if the optional contains a value
- Using StringBuilder::append instead of `+` in some places (to avoid indy concat).
- Changing indexed to for-each loop
- Fixing a link in javadoc
- Removing a spurious semicolon
- Removing an unused method

Jorn

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Commit messages:
 - Applying IntelliJ code cleanups

Changes: https://git.openjdk.java.net/panama-foreign/pull/59/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/59/webrev.00
  Stats: 77 lines in 26 files changed: 0 ins; 49 del; 28 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/59.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/59/head:pull/59

PR: https://git.openjdk.java.net/panama-foreign/pull/59