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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2015-03-04  Pedro Alves  <palves@redhat.com>
+
+	* gdb.threads/thread-specific-bp.exp: Delete "info threads" test.
+
 2015-03-04  Mark Kettenis  <kettenis@gnu.org>
 
 	* gdb.base/foll-fork.exp: Enable on *-*-openbsd*.

gdb/testsuite/gdb.threads/thread-specific-bp.exp

@@ -91,19 +91,6 @@ proc check_thread_specific_breakpoint {mode} {
 	    "Breakpoint .* end .* at .*" \
 	    "continue to end"
 
-	# Force GDB to update the thread list.  Otherwise, depending
-	# on target, GDB may not realize that the start thread has
-	# exited and thus not remove the thread specific breakpoint.
-	set test "thread start is gone"
-	gdb_test_multiple "info threads" $test {
-	    -re "\[0-9\]+.*start.*$gdb_prompt $" {
-		fail $test
-	    }
-	    -re "$gdb_prompt $" {
-		pass $test
-	    }
-	}
-
 	set test "thread-specific breakpoint was deleted"
 	gdb_test_multiple "info breakpoint" $test {
 	    -re "thread $start_thre\n$gdb_prompt $" {