[ios] Remove override of CODE_SIGN_IDENTITY

The variable is now configured from BUILD.gn so should not be
overridden when adding configuration/platforms to the Xcode
project.

Bug: 1094890
Change-Id: I06cef502ebcc3f19b68b91c54e2588b38f411524
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270068
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782982}

ios/build/tools/convert_gn_xcodeproj.py

@@ -120,7 +120,6 @@ def UpdateXcodeProject(project_dir, configurations, root_dir):
       build_config_template = project.objects[value['buildConfigurations'][0]]
       build_config_template['buildSettings']['CONFIGURATION_BUILD_DIR'] = \
           '$(PROJECT_DIR)/../$(CONFIGURATION)$(EFFECTIVE_PLATFORM_NAME)'
-      build_config_template['buildSettings']['CODE_SIGN_IDENTITY'] = ''
 
       value['buildConfigurations'] = []
       for configuration in configurations: