btrace, infrun: replay scheduler locking only depends on to-be-resumed thread

Similar to the parent commit, simplify schedlock_applies() by only
checking the argument thread.

When resuming that thread, GDB will automatically stop replaying its
inferior.  The replay state of other inferiors is not considered by
user_visible_resume_ptid(), so let's not consider them in
schedlock_applies(), either.

Approved-By: Tom Tromey <tom@tromey.com>