Remove sig-network-driver-approvers alias

This previously existed so the dockershim networking stuff had
different approvers than general sig-network stuff, but that code is
gone now, and the remaining kubelet DNS code should be owned by
sig-network in general (and sig-node via inheritance).

OWNERS_ALIASES

@@ -247,9 +247,6 @@ aliases:
     - bobbypage
     - pacoxu
     - bart0sh
-  sig-network-driver-approvers:
-    - dcbw
-    - freehan
   sig-network-approvers:
     - andrewsykim
     - aojea