btrace: honour scheduler-locking for all-stop targets

btrace: honour scheduler-locking for all-stop targets

In all-stop mode, record btrace maintains the old behaviour of an implicit
scheduler-locking on.

Now that we added a scheduler-locking mode to model this old behaviour, we
don't need the respective code in record btrace anymore.  Remove it.

For all-stop targets, step inferior_ptid and continue other threads matching
the argument ptid.  Assert that inferior_ptid matches the argument ptid.

This should make record btrace honour scheduler-locking.

gdb/
	* record-btrace.c (record_btrace_resume): Honour scheduler-locking.

testsuite/
	* gdb.btrace/multi-thread-step.exp: Test scheduler-locking on, step,
	and replay.