gdb: remove target description macros

In my opinion, the target_desc_fetched, current_target_desc and
target_description_filename macros in target-descriptions.c are not very
useful.  I don't think it's useful to hide that they operate on the
current inferior, as everything currently works under the assumption
that the various tdesc commands operate on the current inferior, and I
don't see that changing in the foreseeable future.

This change also avoids having multiple unnecessary calls to
current_inferior and get_tdesc_info per function.

gdb/ChangeLog:

	* target-descriptions.c (struct target_desc_info) <tdesc>:
	Adjust doc.
	(target_desc_fetched): Remove.
	(current_target_desc): Remove.
	(target_description_filename): Remove.
	(target_find_description): Adjust.
	(target_clear_description): Adjust.
	(target_current_description): Adjust.
	(set_tdesc_filename_cmd): Adjust.
	(show_tdesc_filename_cmd): Adjust.
	(unset_tdesc_filename_cmd): Adjust.
	(maint_print_c_tdesc_cmd): Adjust.
	(maint_print_xml_tdesc_cmd): Adjust.

Change-Id: Ibfb581490e949c16d59924e2cac633ede5c26c5b