Remove explicit -Wextra-semi, -Winconsistent-missing-override flags.

They're  enabled via the chromium_code config already these days.
No intended behavior change.

Bug: chromium:926235,chromium:428099
Change-Id: I4f7024ab78cb9672ec3db686e0155b2d835a4790
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564694Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60812}

BUILD.gn

@@ -645,15 +645,7 @@ config("toolchain") {
   }
 
   if (is_clang) {
-    cflags += [
-      "-Wmissing-field-initializers",
-
-      # TODO(thakis): Remove once enabled globally, https://crbug.com/926235
-      "-Wextra-semi",
-
-      # TODO(hans): Remove once http://crbug.com/428099 is resolved.
-      "-Winconsistent-missing-override",
-    ]
+    cflags += [ "-Wmissing-field-initializers" ]
 
     if (v8_current_cpu != "mips" && v8_current_cpu != "mipsel") {
       # We exclude MIPS because the IsMipsArchVariant macro causes trouble.