gdb: remove unused macros from target-debug.h

gdb: remove unused macros from target-debug.h

I noticed some clearly stale entries in target-debug.h (which contained
references to VEC).  I commented everything in target-debug.h and then
uncommented things until it built again, it pointed out a few unused
macros.  Remove them.

gdb/ChangeLog:

	* target-debug.h (target_debug_print_struct_target_ops_p):
	Remove.
	(target_debug_print_async_callback_ftype_p): Remove.
	(target_debug_print_struct_trace_state_variable_p): Remove.
	(target_debug_print_struct_traceframe_info_p): Remove.
	(target_debug_print_VEC__btrace_block_s__pp): Remove.
	(target_debug_print_enum_btrace_format): Remove.
	(target_debug_print_enum_info_proc_what): Remove.
	(target_debug_print_thread_info_pp): Remove.

Change-Id: I7943d21ac6e78751fd7c65a45b345018623ac2f2