Disallow missing field initializers

Change-Id: Ie752121084034432187a5dd7e2950c27994cd13a
diff --git a/BUILD.gn b/BUILD.gn
index 0c9d0a9..ca87c9b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -46,8 +46,6 @@
       "-Wno-implicit-float-conversion",
       "-Wno-implicit-int-conversion",
       "-Wno-implicit-int-float-conversion",
-      # TODO(https://github.com/protocolbuffers/protobuf/pull/8859): Remove this.
-      "-Wno-missing-field-initializers",
       "-Wno-shorten-64-to-32",
       "-Wno-sign-compare",
     ]