RFR: 8263508: Remove dead code in MethodHandleImpl

[Claes Redestad]

Remove unused methods.

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Commit messages:
 - Revert assertSame removal (used from InvokerByteCodeGenerator)
 - Remove dead code in MethodHandleImpl

Changes: https://git.openjdk.java.net/jdk/pull/2969/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2969&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263508
  Stats: 103 lines in 1 file changed: 0 ins; 103 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2969.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2969/head:pull/2969

PR: https://git.openjdk.java.net/jdk/pull/2969