RFR: JDK-8277172: Remove stray comment mentioning instr_size_for_decode_klass_not_null on x64

[Thomas Stuefe]

Trivial cleanup.

https://bugs.openjdk.java.net/browse/JDK-8241825 removed `instr_size_for_decode_klass_not_null()` on x64 but left a comment in place. Remove obsolete comment.

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Commit messages:
 - remove comment

Changes: https://git.openjdk.java.net/jdk/pull/6384/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6384&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277172
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6384.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6384/head:pull/6384

PR: https://git.openjdk.java.net/jdk/pull/6384