No longer pull thread list explicitly

As the result of the patch below, GDB updates thread list when a stop is
presented to user.  The tests don't have to fetch thread list explicitly.

  [PATCH 3/3] Fix non-stop regressions caused by "breakpoints always-inserted off" changes
  https://sourceware.org/ml/gdb-patches/2014-09/msg00734.html

This patch is to remove the test code updating thread list.

Run these three tests many times on arm-linux-gnueabi and x86-linux.
No regressions.

gdb/testsuite:

2014-10-11  Yao Qi  <yao@codesourcery.com>

	* gdb.threads/thread-find.exp: Don't execute command
	"info threads".
	* gdb.threads/attach-into-signal.exp (corefunc): Likewise.
	* gdb.threads/linux-dp.exp: Don't check the condition
	$threads_created equals to zero.