Revert r223709, "[PowerPC]Activate FeatureVSX for the Power target", to unbreak bots.

CodeGen/PowerPC/vsx-p8.ll was failing.

  '+power8-vector' is not a recognized feature for this target (ignoring feature)
  llvm/test/CodeGen/PowerPC/vsx-p8.ll:33:14: error: expected string not found in input
  ; CHECK-REG: lxvw4x 34, 0, 3
               ^
  <stdin>:50:2: note: scanning from here
   .align 3
   ^
  <stdin>:61:2: note: possible intended match here
   lvx 3, 0, 3
   ^