RFR: 8254096: remove jdk.test.lib.Utils::getMandatoryProperty(String) method

[Igor Ignatyev]

Hi all,

could you please review this small and trivial cleanup that removes `getMandatoryProperty` method from
`jdk.test.lib.Utils` as it's unused?

Thanks,
-- Igor

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Commit messages:
 - 8254096: remove jdk.test.lib.Utils::getMandatoryProperty(String) method

Changes: https://git.openjdk.java.net/jdk/pull/533/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=533&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254096
  Stats: 13 lines in 1 file changed: 0 ins; 13 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/533.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/533/head:pull/533

PR: https://git.openjdk.java.net/jdk/pull/533