Remove invalidation support from BTB

Validating the target PC in the pipeline is cheaper than maintaining
the valid bits and control logic to guarantee the BTB won't ever
mispredict branch targets.

rocket/src/main/scala/frontend.scala

@@ -81,7 +81,6 @@ class Frontend(implicit p: Parameters) extends CoreModule()(p) with HasL1CachePa
     btb.io.btb_update := io.cpu.btb_update
     btb.io.bht_update := io.cpu.bht_update
     btb.io.ras_update := io.cpu.ras_update
-    btb.io.invalidate := false
     when (!stall && !icmiss) {
       btb.io.req.valid := true
       s2_btb_resp_valid := btb.io.resp.valid