Remove ability to override mac_sdk_path in flutter/tools/gn (#11013)

This was only required to use an older i386 enabled Mac SDK with a newer version of Xcode for gen_snapshot. This limitation has been patched already.

tools/gn

@@ -259,11 +259,8 @@ def to_gn_args(args):
     gn_args['full_dart_sdk'] = args.full_dart_sdk
 
     if sys.platform == 'darwin':
-      gn_args['mac_sdk_path'] = args.mac_sdk_path
       if args.build_glfw_shell:
         gn_args['build_glfw_shell'] = True
-      if gn_args['mac_sdk_path'] == '':
-        gn_args['mac_sdk_path'] = os.getenv('FLUTTER_MAC_SDK_PATH', '')
 
     return gn_args
 
@@ -326,10 +323,6 @@ def parse_args(args):
                       help='include trained dart2js and dartdevc snapshots. Enable only on steps that create an SDK')
   parser.add_argument('--no-full-dart-sdk', dest='full_dart_sdk', action='store_false')
 
-  parser.add_argument('--mac-sdk-path', default='', type=str,
-                      help='On the mac, the SDK is inferred from the Xcode location unless this flag is specified. ' +
-                      ' Setting the FLUTTER_MAC_SDK_PATH environment variable achieves the same effect.')
-
   parser.add_argument('--ide', default='', type=str,
                       help='The IDE files to generate using GN. Use `gn gen help` and look for the --ide flag to' +
                       ' see supported IDEs. If this flag is not specified, a platform specific default is selected.')