RFR: 8339592: Simplify and remove unused code in ObjectMethods.<clinit>

[Claes Redestad]

A few trivial(?) cleanups to `java.lang.runtime.ObjectMethods`:
- Avoid `MethodType.fromDescriptorString` which needs a classloader
- Use `MethodHandles.zero`
- Remove unused static `MethodHandles` and `MethodTypes`

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Commit messages:
 - 8339592: Simplify and remove unused code in ObjectMethods.<clinit>

Changes: https://git.openjdk.org/jdk/pull/20866/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339592
  Stats: 40 lines in 1 file changed: 0 ins; 21 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/20866.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20866/head:pull/20866

PR: https://git.openjdk.org/jdk/pull/20866