RFR: 8280136: Serial: Remove unnecessary use of ExpandHeap_lock

[Albert Mingkun Yang]

This PR consists of two commits:

1. remove `ExpandHeap_lock` in Serial GC code.
2. rename it to `ParallelExpandHeap_lock` to indicate it's Parallel-GC only.

Test: tier1-6

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Commit messages:
 - rename
 - serial-lock

Changes: https://git.openjdk.java.net/jdk/pull/7124/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7124&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280136
  Stats: 24 lines in 7 files changed: 8 ins; 2 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7124.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7124/head:pull/7124

PR: https://git.openjdk.java.net/jdk/pull/7124