RFR: 8313865: Always true condition in sun.nio.cs.CharsetMapping#readINDEXC2B

[Naoto Sato]

Simple clean-up for removing an unnecessary condition.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/15672/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15672&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313865
  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15672.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15672/head:pull/15672

PR: https://git.openjdk.org/jdk/pull/15672