Don't override ios deployment target in gn (#3751)

Use the value specified in build/config/ios/ios_sdk.gni in the buildroot
repo.

Updates buildroot to 4d1c1fd7103d6daba559e456a6a68e4385bb28f1

DEPS

@@ -54,7 +54,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + '49dc17e37e24ae153a3d2426c958254be1904562',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '4d1c1fd7103d6daba559e456a6a68e4385bb28f1',
 
    # Fuchsia compatibility
    #

tools/gn

@@ -75,7 +75,6 @@ def to_gn_args(args):
         gn_args['target_os'] = 'android'
     elif args.target_os == 'ios':
         gn_args['target_os'] = 'ios'
-        gn_args['ios_deployment_target'] = '7.0'
         gn_args['use_ios_simulator'] = args.simulator
         if not args.simulator:
           aot = True