armv8r64-extras/kas: Remove meta-arm refspec master override

The kirkstone branch has now been created in meta-arm, so the master
override is no longer needed.

Issue-Id: SCM-4386
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Ib4cd88aa7264a42ea6ed27d2140ec062b638fe18