remove unused qf method

remove unused qf method

After the previous patch in the series, nothing uses the "quick"
method find_symbol_file.

This patch removes it.

Tested by rebuilding.

	* dwarf2read.c (dw2_get_primary_filename_reader): Remove.
	(dwarf2_gdb_index_functions): Update.
	* psymtab.c (find_symbol_file_from_partial): Remove.
	(psym_functions): Update.
	* symfile.h (struct quick_symbol_functions) <find_symbol_file>:
	Remove.