Remove Swift plugin Objective-C files

[jmagman]

Both Objective-C apps and Swift apps should be correctly @import/importing plugins as modules. https://github.com/flutter/flutter/pull/42204/files#r333285661

flutter/packages/flutter_tools/lib/src/flutter_plugins.dart

Line 491 in 2ea90ea

@import {{name}};

Remove the Objective-C code in the iOS plugins. This will now (mostly) match the macOS plugin:
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/templates/plugin/macos.tmpl/Classes/pluginClass.swift.tmpl

See conversation at flutter/plugins#6369 (comment).

Plugins are already pretty well integration tested, ex.

flutter/dev/devicelab/bin/tasks/plugin_dependencies_test.dart

Line 281 in 2ea90ea

'ios',

flutter/dev/devicelab/bin/tasks/plugin_test_ios.dart

Line 11 in 2ea90ea

PluginTest('ios', <String>['-i', 'swift', '--platforms=ios']),

Fixes #111928

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[codecov-commenter]

Codecov Report

Merging #113585 (2ea90ea) into master (d2d8902) will not change coverage.
The diff coverage is n/a.

❗ Current head 2ea90ea differs from pull request most recent head c5cf7c6. Consider uploading reports for the commit c5cf7c6 to get more accurate results

@@           Coverage Diff            @@
##           master   #113585   +/-   ##
========================================
  Coverage   91.45%    91.45%           
========================================
  Files         523       523           
  Lines       94639     94639           
========================================
  Hits        86549     86549           
  Misses       8090      8090           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[stuartmorgan]

LGTM!