RFR: 8260605: Various java.lang.invoke cleanups [v2]

[Claes Redestad]

> - Remove unused code
> - Inline and simplify the bootstrap method invocation code (remove pointless reboxing checks etc)
> - Apply pattern matching to make some code more readable

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Missing outstanding changes

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2300/files
  - new: https://git.openjdk.java.net/jdk/pull/2300/files/dc8b586d..68d3475a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2300&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2300&range=00-01

  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2300.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2300/head:pull/2300

PR: https://git.openjdk.java.net/jdk/pull/2300