[foreign-preview] RFR: Remove unused functions and includes

[Jorn Vernee]

This patch removes some unused functions and includes which GCC was issuing warnings about.

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Commit messages:
 - Remove unused functions and includes

Changes: https://git.openjdk.java.net/panama-foreign/pull/648/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=648&range=00
  Stats: 19 lines in 1 file changed: 0 ins; 19 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/648.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/648/head:pull/648

PR: https://git.openjdk.java.net/panama-foreign/pull/648