Revert "Don't block while runnable in getReferent()"

This reverts commit 1d9d3587dafb7078082b4d4058eea753c13ef94f.

Reason for revert: Likely cause of build failures like https://android-build.googleplex.com/builds/submitted/7611067/art-jit/latest/logs

Change-Id: I008de0d4b3976b8380935cd9a389c77e3b396274
diff --git a/runtime/gc/reference_processor.cc b/runtime/gc/reference_processor.cc
index 12b1337..e34d140 100644
--- a/runtime/gc/reference_processor.cc
+++ b/runtime/gc/reference_processor.cc
@@ -155,7 +155,6 @@
       started_trace = true;
       start_millis = MilliTime();
     }
-    ScopedThreadSuspension sts(self, ThreadState::kSuspended);
     condition_.WaitHoldingLocks(self);
   }
   if (started_trace) {