[build] Turn off -Wdeprecated-copy warnings

It fires with newer versions of Clang (see bug).

Bug: chromium:1201858
Change-Id: Ie55b39a0422a5f45b4eb6bdfcc77114e1ef554bb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2846836
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>

BUILD.gn

@@ -209,7 +209,6 @@ config("extra_warnings") {
     if (!use_xcode_clang) {
       # Mac catalyst uses a clang version that doesn't have these.
       cflags += [
-        "-Wdeprecated-copy",
         "-Wsuggest-destructor-override",
         "-Wsuggest-override",
       ]