Remove use_modular_headers! from Swift Podfiles (flutter#156257)

We added `use_modular_headers!` to our `Podfile`s as we originally planned to phase out `use_frameworks!` (see flutter#42204). However, our plans have now changed and we are instead phasing out CocoaPods entirely in favor of Swift Package Manager.

CocoaPods's `use_frameworks!` and `use_modular_headers!` are two different overlapping options that should not be used together. This change removes the `use_modular_headers!` from the macOS `Podfile` and the iOS Swift `Podfile` (the iOS Objective-C template was recently deprecated flutter#155867).

This change only affects _new_ Flutter apps. This change does not include an automatic migration as that could break existing apps. Instead, users are encouraged to migrate from CocoaPods to Swift Package Manager.

flutter#156259

Author: loic-sharma

dev/benchmarks/complex_layout/ios/Podfile

@@ -29,7 +29,6 @@ flutter_ios_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_ios_pods File.dirname(File.realpath(__FILE__))
 end

dev/benchmarks/complex_layout/macos/Podfile

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
 end

dev/benchmarks/macrobenchmarks/macos/Podfile

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
 end

dev/integration_tests/channels/macos/Podfile

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
   target 'RunnerTests' do

dev/integration_tests/flavors/macos/Podfile

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
   target 'RunnerTests' do

dev/integration_tests/flutter_gallery/macos/Podfile

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
 

dev/integration_tests/ios_app_with_extensions/ios/Podfile

@@ -29,15 +29,13 @@ flutter_ios_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_ios_pods File.dirname(File.realpath(__FILE__))
 end
 
 target 'watch Extension' do
   platform :watchos
   use_frameworks!
-  use_modular_headers!
 
   pod 'EFQRCode', '6.0'
 end

dev/integration_tests/ui/macos/Podfile

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
 end

examples/api/macos/Podfile

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
 end

packages/flutter_tools/templates/cocoapods/Podfile-ios-swift

@@ -29,7 +29,6 @@ flutter_ios_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_ios_pods File.dirname(File.realpath(__FILE__))
   target 'RunnerTests' do

packages/flutter_tools/templates/cocoapods/Podfile-macos

@@ -28,7 +28,6 @@ flutter_macos_podfile_setup
 
 target 'Runner' do
   use_frameworks!
-  use_modular_headers!
 
   flutter_install_all_macos_pods File.dirname(File.realpath(__FILE__))
   target 'RunnerTests' do