Fix bad auto merge conflict resolution in CC

We should not be clearing the region space since:
https://android-review.googlesource.com/#/c/294708/

Bug: 12687968

Test: test-art-host CC baker
Change-Id: I78050c61c8cdcc0b777d3324ba3f7ded80b2cf52
diff --git a/runtime/gc/collector/concurrent_copying.cc b/runtime/gc/collector/concurrent_copying.cc
index 8bb90e1..6dfab8b 100644
--- a/runtime/gc/collector/concurrent_copying.cc
+++ b/runtime/gc/collector/concurrent_copying.cc
@@ -2335,7 +2335,6 @@
     TimingLogger::ScopedTiming split("ClearRegionSpaceCards", GetTimings());
     // We do not currently use the region space cards at all, madvise them away to save ram.
     heap_->GetCardTable()->ClearCardRange(region_space_->Begin(), region_space_->Limit());
-    region_space_ = nullptr;
   }
   {
     MutexLock mu(self, skipped_blocks_lock_);