gdbserver: Remove traces of ancient Hc handling

gdbserver: Remove traces of ancient Hc handling

Back in commit f0db101d9897 ("gdbserver: don't pick a random thread if
the current thread dies"), a couple years ago, the last references to
set_desired_thread(0) [select the Hc thread] were removed, and all the
remaining calls to set_desired_thread pass '1', meaning general
thread.  This means we can simplify set_desired_thread.

gdb/gdbserver/ChangeLog:
2017-09-21  Pedro Alves  <palves@redhat.com>

	* server.c (gdb_read_memory, handle_status, process_serial_event)
	(handle_serial_event, handle_target_event): Adjust to
	set_desired_thread prototype change.
	* target.c (set_desired_thread): Remove 'use_general' parameter
	and adjust.
	* target.h (set_desired_thread): Remove 'use_general' parameter.