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.