Remove ATK_DISABLE_DEPRECATION_WARNINGS.

Change-Id: I27698aa13afe8a065acd8eaf5a89ac6dea762aa4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2136467Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756771}

build/config/linux/atk/BUILD.gn

@@ -30,11 +30,5 @@ pkg_config("atk") {
   defines = [
     "ATK_LIB_DIR=\"$atk_lib_dir\"",
     "USE_ATK_BRIDGE",
-
-    # The AtkValue interface methods we implement and test have been deprecated
-    # in favor of new API. But that new API cannot be used until corresponding
-    # support has been added to AT-SPI2 and we stop supporting earlier versions
-    # of ATK (i.e. < 2.12).
-    "ATK_DISABLE_DEPRECATION_WARNINGS",
   ]
 }