RFR: 8275975: Remove dead code in ciInstanceKlass

[Tobias Hartmann]

I've noticed some dead code in `ciInstanceKlass` (and also in `ciArrayKlass`). Mostly leftovers from [JDK-8237767](https://bugs.openjdk.java.net/browse/JDK-8237767).

Thanks,
Tobias

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Commit messages:
 - 8275975: Remove dead code in ciInstanceKlass

Changes: https://git.openjdk.java.net/jdk/pull/6118/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6118&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275975
  Stats: 15 lines in 3 files changed: 0 ins; 15 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6118.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6118/head:pull/6118

PR: https://git.openjdk.java.net/jdk/pull/6118