RFR: 8255049: Remove support for the hsdis decode_instructions entry point in hotspot

[Claes Redestad]

This patch drops support in hotspot for hsdis plugins which only include the old decode_instructions endpoint.

The decode_instructions entry point in hsdis was replaced by decode_instructions_virtual[1], with support later added to allow old hsdis plugins to work, at least for the duration of JDK 8. Dropping the backwards compatibility means you'll need a hsdis built from JDK 8 sources or later, which seems a reasonable requirement at this point.

It's unclear if a CSR request is needed.

[1] https://github.com/openjdk/jdk/commit/22544e7a7c72e8779355df963e49e846f9458ce4
[2] https://github.com/openjdk/jdk/commit/a9c40e9df41ee06adcd7fff951dd36b6c093a24b

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Commit messages:
 - Remove unused declarations
 - Merge branch 'master' into cleanup_hsdis_entrypoints
 - Remove _decode_instructions support

Changes: https://git.openjdk.java.net/jdk/pull/807/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=807&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255049
  Stats: 39 lines in 2 files changed: 0 ins; 31 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/807.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/807/head:pull/807

PR: https://git.openjdk.java.net/jdk/pull/807