RFR: 8280136: Serial: Remove unnecessary use of ExpandHeap_lock [v2]

[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:

  review

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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/16874ed0..8e98e826

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7124&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7124&range=00-01

  Stats: 40 lines in 6 files changed: 5 ins; 25 del; 10 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