Fix gdb.threads/thread-specific-bp.exp race

Gary stumbled on this:

 (gdb) PASS: gdb.threads/thread-specific-bp.exp: all-stop: continue to end
 info threads
   Id   Target Id         Frame
 * 1    Thread 0x7ffff7fdb700 (LWP 13717) "thread-specific" end () at /home/gary/work/archer/startswith/src/gdb/testsuite/gdb.threads/thread-specific-bp.c:29
 (gdb) FAIL: gdb.threads/thread-specific-bp.exp: all-stop: thread start is gone
 info breakpoint

The problem is that "...archer/startswith/src..." has a "start" in it,
which matches the too-lax regex in the test.

Rather than tweaking the regex, we can just remove the whole "info
threads", like we removed similar ones in other files -- GDB nowadays
does this implicitly already, so things should work without it.  Thus
removing this even improves testing here a bit.

gdb/testsuite/ChangeLog:
2015-03-04  Pedro Alves  <palves@redhat.com>

	* gdb.threads/thread-specific-bp.exp: Delete "info threads" test.