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

[David Holmes]

On Sat, 18 Feb 2023 15:35:35 GMT, Afshin Zafari <duke at openjdk.org> wrote:

> The function definition and declaration are removed.
> 
> ### Test
> mach5 tiers 1-5

Looks good and trivial.

This function has been unused since JDK-8079408 was fixed. I see now that `print_flag_error_message_if_needed` in `writeableFlags.cpp` basically replaced the need to map the flag errors directly to strings.

Thanks.

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Marked as reviewed by dholmes (Reviewer).

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