//build/config/mac/base_rules.gni was split up.

We included it in build/mac/BUILD.gn, but I don't see why. For now just
removed the include.

Chromium change:

https://chromium.googlesource.com/chromium/src.git/+/0f1c9c0557f17799ba9fe3666b583919646680c7

commit 0f1c9c0557f17799ba9fe3666b583919646680c7
Author: Sylvain Defresne <sdefresne@chromium.org>
Date:   Thu Mar 11 14:40:07 2021 +0000

    [apple] Split //build/config/mac/base_rules.gni

    The templates defined in that file were used to build for both ios
    and macOS OSes. Move each template to a separate file (named after
    the template defined) and put the files in //build/apple (which is
    the location of template shared between all Apple platforms).

    This is a reland of https://crrev.com/c/2740920 with both the fix
    included in https://crrev.com/c/2743765 and the addition of missing
    import of //build/config/mac/mac_sdk.gni to get the definition of
    mac_bin_path in //chrome/installer/mac/BUILD.g

    Bug: 635745

build/mac/BUILD.gn

@@ -1,6 +1,5 @@
 import("//brave/build/config.gni")
 import("//brave/build/mac/config.gni")
-import("//build/config/mac/base_rules.gni")
 import("//build/util/branding.gni")
 import("//build/util/version.gni")
 import("//chrome/common/features.gni")