Remove some unused variables in dwarf2read.c

Remove some unused variables in dwarf2read.c

Most of them are obvious.  The ones in dwarf2_record_block_ranges are
less obvious, because it is a bit suspicious to have that many
variables unused.  But after inspection, it seems like it dates from
commit 5f46c5a54825 ("Code cleanup: Split dwarf2_ranges_read to a
callback"), where dwarf2_record_block_ranges was made to use
dwarf2_ranges_process, which contains the same functionality.

gdb/ChangeLog:

	* dwarf2read.c (create_signatured_type_table_from_debug_names):
	Remove unused variables.
	(dw2_map_symtabs_matching_filename): Likewise.
	(dwarf2_record_block_ranges): Likewise.
	(dwarf2_read_addr_index): Likewise.
	(follow_die_offset): Likewise.