Remove objfile parameter from abbrev_table::read

In a longer series that I am working on, I needed to remove the
objfile parameter from abbrev_table::read.  It seemed to me that this
was a simple and relatively harmless patch, so I'm sending it now.

gdb/ChangeLog
2020-11-05  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_cutu_die_from_dwo)
	(cutu_reader::cutu_reader, cutu_reader::cutu_reader)
	(build_type_psymtabs_1): Update.
	* dwarf2/abbrev.h (struct abbrev_table): Remove objfile
	parameter.
	* dwarf2/abbrev.c (abbrev_table::read): Remove objfile parameter.
	Don't read section.  Add assert.