RFR: 8366155: Serial: Obsolete PretenureSizeThreshold

[Albert Mingkun Yang]

Remove obj-size checking before young/old-gen allocation -- an allocation succeeds iff there is enough free space.

This simplifies the allocation logic:
1. before attempting gc: `mem_allocate_work` does young-gen allocation then old-gen-no-expansion allocation
2. after gc (still inside gc safepoint): `satisfy_failed_allocation` does young-gen allocation old-gen-with-expansion allocation

Test: tier1-3

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Commit messages:
 - sgc-should-allocate

Changes: https://git.openjdk.org/jdk/pull/26941/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26941&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366155
  Stats: 86 lines in 7 files changed: 3 ins; 75 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/26941.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26941/head:pull/26941

PR: https://git.openjdk.org/jdk/pull/26941