RFR: 8309265: Serial GC: remove the related code about GC overheap limit

[Guoxiong Li]

Hi all,

This patch removes the unimplemented GC overhead limit related code in serial GC, mainly in `genCollectedHeap`.
It seems a leftover when removing the CMS.

Thanks for the review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8309265

Changes: https://git.openjdk.org/jdk/pull/14258/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14258&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309265
  Stats: 30 lines in 2 files changed: 0 ins; 26 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14258.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14258/head:pull/14258

PR: https://git.openjdk.org/jdk/pull/14258