Remove `EventLoopExtIOS::idiom` and `ios::Idiom`

[madsmtm]

Introduced in #871 with unclear motivation.

@mtak- could you provide context for why Winit has this? Is it just for convenience?

• [ ] Tested on all platforms changed
• [x] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality
• [x] Updated feature matrix, if new features were added or implemented