Generation::expand_and_allocate has unused "parallel" argument

Generation::expand_and_allocate has a third argument, called "parallel", which defaults to false. There are no callers that provide that argument, so it always has the default value.

DefNewGeneration just ignores that argument.

TenuredGeneration conditionalizes its behavior based on that argument. The parallel case can be deleted, since it will never be used.

[This is probably a leftover from CMS support. The Generation class is now only used by SerialGC.]