Revert "Ignore warning for instantiation after specialization."

This reverts commit 445584d3d1f8e3a9a7d12eded3b2dce5f8691d8c.

Reason for revert: <INSERT REASONING HERE>

Change-Id: I911049f64c4e963cfa47fd8bc7f16ed14e25b057
diff --git a/runtime/base/arena_allocator.cc b/runtime/base/arena_allocator.cc
index 1caf0c0..61e0aab 100644
--- a/runtime/base/arena_allocator.cc
+++ b/runtime/base/arena_allocator.cc
@@ -144,11 +144,8 @@
   }
 }
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Winstantiation-after-specialization"
 // Explicitly instantiate the used implementation.
 template class ArenaAllocatorStatsImpl<kArenaAllocatorCountAllocations>;
-#pragma GCC diagnostic pop
 
 void ArenaAllocatorMemoryTool::DoMakeDefined(void* ptr, size_t size) {
   MEMORY_TOOL_MAKE_DEFINED(ptr, size);