RFR: 8313403: Remove unused 'mask' field from JFormattedTextField

[Phil Race]

On Mon, 31 Jul 2023 10:26:44 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

> The private field `mask` is never used in `JFormattedTextField`.
> 
> I couldn't find any usages of the field, including JNI. Therefore, it is safe to remove.

Marked as reviewed by prr (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/15088#pullrequestreview-1583531782