Garbage collect remote.c:remote_async_terminal_ours_p

Garbage collect remote.c:remote_async_terminal_ours_p

remote.c:remote_async_terminal_ours_p stopped being useful after
048094accce2 ("target remote: Don't rely on immediate_quit (introduce
quit handlers)") and commit 41fd2b0f5d95 ("Make input_fd be per UI"),
which turned remote's terminal_inferior/ours methods into nops.

gdb/ChangeLog:
2017-10-26  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_async_terminal_ours_p): Delete.
	(remote_open_1, remote_terminal_inferior, remote_terminal_ours):
	Remove references to 'remote_async_terminal_ours_p'.