Revert "Disable -Wtautological-unsigned-zero-compare"

This reverts commit 66db2192b5067a53a3d901d56f4b8155fb7e9b85.

Reason for revert: This is only supported by the new toolchain.

Change-Id: I0e6a6950118bfa00ba6d111fa56d68839e7d8864
diff --git a/BUILD.gn b/BUILD.gn
index 66f6f47..fd39331 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -13,7 +13,6 @@
     "-Wall",
     "-Wextra",
     "-Wno-missing-field-initializers",
-    "-Wno-tautological-unsigned-zero-compare",
     "-Wno-unused-parameter",
   ]
 }