Replace sort_tu_by_abbrev_offset with operator<

I noticed that sort_tu_by_abbrev_offset only has a single caller.  It
seemed simpler to replace it with an implementation of operator<
instead.

2021-05-17  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (tu_abbrev_offset::operator<): New method.
	(sort_tu_by_abbrev_offset): Remove.
	(build_type_psymtabs): Update.