refactor!: Easy deprecation cleanups

[dpogue]

Platforms affected

iOS

Motivation and Context

Closes #1428.

Description

• Added deprecation annotations to things we want to discourage people from using (some of these can be removed in Cordova iOS 9)
• Remove some conditionals for iOS versions pre-11 and dead code
• Change CDVPlugin's viewController from a UIViewController* to a CDVViewController*
• Move a webview engine-related delegate to a header that makes more sense

Testing

Unit tests pass.

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [x] I added automated test coverage as appropriate for this change
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.35%. Comparing base (c8e126d) to head (7c10750).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1455 +/- ## ======================================= Coverage 78.35% 78.35% ======================================= Files 16 16 Lines 1825 1825 ======================================= Hits 1430 1430 Misses 395 395 ```

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