RFR: CODETOOLS-7903303: remove local copy of RuntimeVersion

[Jonathan Gibbons]

Please review an almost trivial change, to use the standard `Runtime.Version` class now that `jtreg` assumes JDK >= 11, and to remove the local "cached" copy in `RuntimeVersion`.

-------------

Commit messages:
 - CODETOOLS-7903303: remove local copy of RuntimeVersion

Changes: https://git.openjdk.org/jtreg/pull/119/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=119&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903303
  Stats: 697 lines in 2 files changed: 0 ins; 696 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/119.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/119/head:pull/119

PR: https://git.openjdk.org/jtreg/pull/119