Make unlink_objfile and put_objfile_before static

Make unlink_objfile and put_objfile_before static

I noticed an obsolete comment just before unlink_objfile, and then I
noticed that both unlink_objfile and put_objfile_before could be
static.  This patch makes these changes, and also moves unlink_objfile
earlier, so that a forward declaration is not needed.

Tested by rebuilding.

gdb/ChangeLog
2019-10-20  Tom Tromey  <tom@tromey.com>

	* objfiles.h (unlink_objfile, put_objfile_before): Don't declare.
	* objfiles.c (unlink_objfile): Move earlier.  Now static.  Remove
	obsolete comment.
	(put_objfile_before): Now static.

Change-Id: I1b5927a60fd1cc59bfc9c6761f61652a01ef13e0