RFR: 8300654: Remove JVMFlag::flag_error_str as it is unused

[Afshin Zafari]

The function definition and declaration are removed.

### Test
mach5 tiers 1-5

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Commit messages:
 - 8300654: Remove JVMFlag::flag_error_str as it is unused

Changes: https://git.openjdk.org/jdk/pull/12636/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12636&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300654
  Stats: 17 lines in 2 files changed: 0 ins; 16 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12636.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12636/head:pull/12636

PR: https://git.openjdk.org/jdk/pull/12636