Remove unused file_string parameter in gdb_disassembly

The file_string parameter was added in 8f0eea0e (sorry, no title back
then) and has never actually been used.

gdb/ChangeLog:

	* disasm.h (gdb_disassembly): Remove file_string parameter.
	* disasm.c (gdb_disassembly): Likewise.
	* cli/cli-cmds.c (print_disassembly): Adapt.
	* mi/mi-cmd-disas.c (mi_cmd_disassemble): Likewise.
	* stack.c (do_gdb_disassembly): Likewise.