RFR: 8245256: Cleanup OpenGL references in Metal documentation

[Jayathirth D V]

This also removes good amount of stub and unused code which was copy pasted from OpenGL.

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Commit messages:
 - Native level cleanup
 - Java level cleanup

Changes: https://git.openjdk.java.net/lanai/pull/150/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lanai&pr=150&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245256
  Stats: 697 lines in 26 files changed: 0 ins; 608 del; 89 mod
  Patch: https://git.openjdk.java.net/lanai/pull/150.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/150/head:pull/150

PR: https://git.openjdk.java.net/lanai/pull/150