Remove pg_rewind's private logging.h/logging.c files.

Remove pg_rewind's private logging.h/logging.c files.

The existence of these files became rather confusing with the
introduction of a widely-known logging.h header in commit cc8d41511.
(Indeed, there's already some duplicative #includes here, perhaps
betraying such confusion.)  The only thing left in them, after that
commit, is a progress-reporting function that's neither general-purpose
nor tied in any way to other logging infrastructure.  Hence, let's just
move that function to pg_rewind.c, and get rid of the separate files.

Discussion: https://postgr.es/m/3971.1557787914@sss.pgh.pa.us