re-enable txn auto gc

fixes cockroachdb#2717 (ok since cockroachdb#2799 has landed).

storage/replica.go

@@ -87,10 +87,7 @@ var TestingCommandFilter func(roachpb.Request, roachpb.Header) error
 // upon EndTransaction if they only have local intents (which can be
 // resolved synchronously with EndTransaction). Certain tests become
 // simpler with this being turned off.
-// TODO(tschottdorf): Push after removal can happen if txn is not
-// single-roundtrip, and pusher will recreate a PENDING entry, getting
-// stuck. Since txn gc story is not done yet (#2062), disabled for now.
-var txnAutoGC = false
+var txnAutoGC = true
 
 // raftInitialLogIndex is the starting point for the raft log. We bootstrap
 // the raft membership by synthesizing a snapshot as if there were some