RFR: 8318104: macOS 10.13 check in TabButtonAccessibility.m can be removed

[Abhishek Kumar]

Since[ JDK-8317970](https://bugs.openjdk.org/browse/JDK-8317970) is integrated to make macOS 11 the minimum version to build jdk, the 10.13 check is not needed and can be removed from TabButtonAccessibility.m file.

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Commit messages:
 - macOS 10.13 check can be removed

Changes: https://git.openjdk.org/jdk/pull/16218/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318104
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16218.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16218/head:pull/16218

PR: https://git.openjdk.org/jdk/pull/16218