Skia can now build with 10.11 as the min deployment version (#23717)

https://bugs.chromium.org/p/skia/issues/detail?id=11160
was blocking this from happening, now that its addresses we can
work on a build to support both metal and opengl.

tools/gn

@@ -265,7 +265,6 @@ def to_gn_args(args):
       # Skia has Metal support on macOS version >= 10.14.
       MACOS_SKIA_METAL_SUPPORTED_MIN_VERSION = '10.14'
       gn_args['mac_sdk_min'] = MACOS_SKIA_METAL_SUPPORTED_MIN_VERSION
-      gn_args['mac_deployment_target'] = MACOS_SKIA_METAL_SUPPORTED_MIN_VERSION
 
     if args.enable_vulkan:
       # Enable vulkan in the Flutter shell.