RFR (XS) 8243460: Shenandoah: purge init_update_refs_prepare counter

[Aleksey Shipilev]

Simple RFE:
  https://bugs.openjdk.java.net/browse/JDK-8243460

After JDK-8243291, init_update_refs_prepare counter is effectively zero on most paths and does not
carry its weight. Removing it unclutters the logs.

Fix:

diff -r 4103af83d98e src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp        Thu Apr 23 12:14:58 2020 +0900
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp        Thu Apr 23 07:50:01 2020 +0200
@@ -2507,8 +2507,5 @@
   set_update_refs_in_progress(true);

-  {
-    ShenandoahGCSubPhase phase(ShenandoahPhaseTimings::init_update_refs_prepare);
-    _update_refs_iterator.reset();
-  }
+  _update_refs_iterator.reset();

   if (ShenandoahPacing) {
diff -r 4103af83d98e src/hotspot/share/gc/shenandoah/shenandoahPhaseTimings.hpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahPhaseTimings.hpp        Thu Apr 23 12:14:58 2020 +0900
+++ b/src/hotspot/share/gc/shenandoah/shenandoahPhaseTimings.hpp        Thu Apr 23 07:50:01 2020 +0200
@@ -91,5 +91,4 @@
   f(init_update_refs,                               "Pause Init  Update Refs (N)")     \
   f(init_update_refs_retire_gclabs,                 "  Retire GCLABs")                 \
-  f(init_update_refs_prepare,                       "  Prepare")                       \
                                                                                        \
   f(final_update_refs_gross,                        "Pause Final Update Refs (G)")     \

-- 
Thanks,
-Aleksey