RFR: 8268647: Generation::expand_and_allocate has unused "parallel" argument

[Kim Barrett]

Please review this simplification of the Generation class and the classes
derived from it.  There are no callers of expand_and_allocate that pass the
third argument explicitly, so its value is always the default (false) value.

This change removes the argument and updates the two implementations of this
virtual function accordingly.

[The "parallel" argument is probably a leftover from CMS support.]

Testing:
mach5 tier1.

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Commit messages:
 - remove parallel argument

Changes: https://git.openjdk.java.net/jdk/pull/4485/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4485&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268647
  Stats: 42 lines in 5 files changed: 0 ins; 31 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4485.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4485/head:pull/4485

PR: https://git.openjdk.java.net/jdk/pull/4485