Remove two trivial functions from dwarf2/read.c

Remove two trivial functions from dwarf2/read.c

This removes dw2_map_matching_symbols and dw2_expand_symtabs_matching,
merging them with their sole trivial callers.

gdb/ChangeLog
2021-03-31  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf2_gdb_index::map_matching_symbols): Merge
	with dw2_map_matching_symbols.
	(dwarf2_gdb_index::expand_symtabs_matching): Merge with
	dw2_expand_symtabs_matching.