Remove find_inferior usage for thread_search

Remove find_inferior usage for thread_search

Replace it with for_each_thread.  While at it, we can inline the
callback code.  One little change is that I am using the
prev_general_thread variable instead of current_gen_ptid, since they
should have the same value.

gdb/gdbserver/ChangeLog:

	* target.c (struct thread_search): Remove.
	(thread_search_callback): Remove.
	(prepare_to_access_memory): Use for_each_thread instead of
	find_inferior.  Inline code from thread_search_callback.