RFR: 8311086: Remove jtreg/gc/startup_warnings

[Leo Korinth]

It does not seem important to check that certain GCs are not deprecated.

The test originally tested that we created deprecation warnings on features that are now removed. What is left is not worth having IMO.

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Commit messages:
 - 8311086: Remove jtreg/gc/startup_warnings

Changes: https://git.openjdk.org/jdk/pull/14709/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311086
  Stats: 202 lines in 5 files changed: 0 ins; 202 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14709.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14709/head:pull/14709

PR: https://git.openjdk.org/jdk/pull/14709