Don't check HAVE_UNISTD_H

I noticed some spots that were checking HAVE_UNISTD_H.  There is no
need to do this, as <unistd.h> is unconditionally included in many
places in gdb.  This sort of cleanup was done once before, in 2013:

    2013-07-01  Pedro Alves  <palves@redhat.com>

	    * defs.h: Don't check HAVE_UNISTD_H before including <unistd.h>.
	    (STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO): Delete.
	    * tracepoint.c: Don't check HAVE_UNISTD_H before including
	    <unistd.h>.

HAVE_UNISTD_H seems to come from gnulib, so there are still mentions
of it in the source.

gdb/ChangeLog
2018-10-01  Tom Tromey  <tom@tromey.com>

	* unittests/scoped_mmap-selftests.c: Don't check HAVE_UNISTD_H.
	* unittests/scoped_fd-selftests.c: Don't check HAVE_UNISTD_H.
	* common/scoped_fd.h: Don't check HAVE_UNISTD_H.