thread.c: cleanup breakpoint deletion

thread.c: cleanup breakpoint deletion

A little refactoring to reduce duplicate code.

gdb/
2014-10-15  Pedro Alves  <palves@redhat.com>

	* thread.c (delete_thread_breakpoint): New function.
	(delete_step_resume_breakpoint)
	(delete_exception_resume_breakpoint): Use it.
	(delete_at_next_stop): New function.
	(clear_thread_inferior_resources): Use delete_at_next_stop.