drop dhcp auditallow

toolbox_exec is executed by dhcpcd-run-hooks.

Addresses the following auditallow log spam:

  avc: granted { read open } for comm="dhcpcd-run-hook" path="/system/bin/toybox" dev="dm-0" ino=650 scontext=u:r:dhcp:s0 tcontext=u:object_r:toolbox_exec:s0 tclass=file
  avc: granted { read } for comm="setprop" name="toybox" dev="dm-0" ino=650 scontext=u:r:dhcp:s0 tcontext=u:object_r:toolbox_exec:s0 tclass=file
  avc: granted { execute } for comm="dhcpcd-run-hook" name="toybox" dev="dm-0" ino=650 scontext=u:r:dhcp:s0 tcontext=u:object_r:toolbox_exec:s0 tclass=file
  avc: granted { execute_no_trans } for comm="dhcpcd-run-hook" path="/system/bin/toybox" dev="dm-0" ino=650 scontext=u:r:dhcp:s0 tcontext=u:object_r:toolbox_exec:s0 tclass=file
  avc: granted { read execute } for comm="setprop" path="/system/bin/toybox" dev="dm-0" ino=639 scontext=u:r:dhcp:s0 tcontext=u:object_r:toolbox_exec:s0 tclass=file

Change-Id: Ib83c291961a5573397830032a67a2f6861ae2e71