Remove unused build override

build_overrides/build.gni

Index: build_overrides/build.gni
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index 122ec63f7519397420989e597076e8c32b4f545e..d0939fc9702fbbcc47df2ebb9ecefac35614c0d6 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -2,12 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# Several dependencies of Chrome require a different min sdk and/or deployment
-# target when built as stand-alone projects. If this is ever not the case, these
+# Several dependencies of Chrome require a different min sdk
+# when built as stand-alone projects. If this is ever not the case, these
 # variables can be removed.
 # See https://bugs.chromium.org/p/webrtc/issues/detail?id=5453.
 mac_sdk_min_build_override = "10.10"
-mac_deployment_target_build_override = "10.9"
 
 # Variable that can be used to support multiple build scenarios, like having
 # Chromium specific targets in a client project's GN file etc.