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

[Alexey Ivanov]

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.

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Commit messages:
 - 8313403: Remove unused 'mask' field from JFormattedTextField

Changes: https://git.openjdk.org/jdk/pull/15088/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15088&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313403
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15088.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15088/head:pull/15088

PR: https://git.openjdk.org/jdk/pull/15088