RFR: 8348117: The two-argument overload of SignatureHandlerLibrary::add is not used

[David Holmes]

The  `SignatureHandlerLibrary::add` two-arg API was added to support closed code under [JDK-8087333](https://bugs.openjdk.org/browse/JDK-8087333) in JDK 9. That closed code was removed again, still in JDK 9, as part of JEP 297 ([JDK-8168503](https://bugs.openjdk.org/browse/JDK-8168503)) which unified the closed and open ARM ports - leaving the API unused.

There were other leftovers from the same set of changes so they are cleaned up as well - nothing significant, just unnecessary friend declarations, comments, and one other method.

Testing:
 - all builds in tiers 1-5

Thanks.

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Commit messages:
 - 8348117: The two-argument overload of SignatureHandlerLibrary::add is not used

Changes: https://git.openjdk.org/jdk/pull/23207/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23207&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348117
  Stats: 40 lines in 9 files changed: 0 ins; 40 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23207.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23207/head:pull/23207

PR: https://git.openjdk.org/jdk/pull/23207