RFR: 8344186: Cleanup sun.net.www.MimeTable after JEP 486 integration

[Jaikiran Pai]

Can I please get a review of this change which removes the usages of `SecurityManager` and calls to `AccessController.doPrivileged()` from the `sun.net.www.MimeTable` class?

No new tests have been introduced and existing tests in tier1 and tier2 continue to pass with this change.

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Commit messages:
 - 8344186: Cleanup sun.net.www.MimeTable after JEP 486 integration

Changes: https://git.openjdk.org/jdk/pull/22137/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22137&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344186
  Stats: 37 lines in 1 file changed: 0 ins; 25 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/22137.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22137/head:pull/22137

PR: https://git.openjdk.org/jdk/pull/22137