[macOS] Remove FlutterApplication class

[cbracken]

The need for the FlutterApplication NSApplication subclass was eliminated in 8679b67. The relevant app termination logic was moved into FlutterAppDelegate in that patch. Removing FlutterApplication needs to be done in three stages:

1. Migrate logic to FlutterAppDelegate. [macOS] Handle termination in FlutterAppDelegate #40929.
2. Update Flutter tool template. Update migrator to migrate all apps using FlutterApplication back to NSApplication. [macOS] Remigrate principal class to NSApplication flutter#124173.
3. Eliminate FlutterApplication.h header since all references to it have now been removed. [macOS] Remove FlutterApplication class #40939.

No tests since, as described above, this is the removal of unused (as of step 2 above) code.

Issue: flutter/flutter#30735

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 and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[flutter-dashboard]

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Changes reported for pull request #40939 at sha 158598a

[gspencergoog]