Further cleanup of signal-while-stepping-over-bp-other-thread.c.

This test now uses pthread_kill instead of the host's kill command, so
no longer need to block signals, or store the the inferior's PID.

gdb/testsuite/
2014-03-20  Pedro Alves  <palves@redhat.com>

	* gdb.threads/signal-while-stepping-over-bp-other-thread.c (pid):
	Delete.
	(block_signals, unblock_signals): Delete.
	(child_function_2, main): Remove references to deleted variable
	and functions.