* source.c (forget_cached_source_info): Clear last_source_visited.

	* source.c (forget_cached_source_info): Clear last_source_visited.
	(init_last_source_visited): Delete.
	(directory_command): Do not clear last_source_visited.  Call
	forget_cached_source_info only if required.
	(unset_substitute_path_command, set_substitute_path_command): Call
	forget_cached_source_info.
	* mi/mi-cmd-env.c (mi_cmd_env_dir): Do not call
	init_last_source_visited.
	* defs.h (init_last_source_visited): Delete declaration.