RFR: 8280136: Serial: Remove unnecessary use of ExpandHeap_lock [v4]

[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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  lower case

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7124/files
  - new: https://git.openjdk.java.net/jdk/pull/7124/files/fa5dcce9..d5a2a9ca

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7124&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7124&range=02-03

  Stats: 11 lines in 2 files changed: 0 ins; 0 del; 11 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