Ignore the result of pthread_kill in ubf_wakeup_thread

	After an upgrade to Ruby 3.3.0, I experienced reproducible production crashes
	of the form:

	[BUG] pthread_kill: No such process (ESRCH)

	This is the only pthread_kill call in Ruby. The result of pthread_kill was
	previously ignored in Ruby 3.2 and below. Checking the result was added in
	be1bbd5b (MaNy).

	I have not yet been able to create a minimal self-contained example,
	but it should be safe to remove the checks.