Remove pg_dump/parallel.c's useless "aborting" flag.

Remove pg_dump/parallel.c's useless "aborting" flag.

This was effectively dead code, since the places that tested it could not
be reached after we entered the on-exit-cleanup routine that would set it.
It seems to have been a leftover from a design in which error abort would
try to send fresh commands to the workers --- a design which could never
have worked reliably, of course.  Since the flag is not cross-platform, it
complicates reasoning about the code's behavior, which we could do without.

Although this is effectively just cosmetic, back-patch anyway, because
there are some actual bugs in the vicinity of this behavior.

Discussion: <15583.1464462418@sss.pgh.pa.us>