Remove "nested" result enums for some older endpoints

[Nerixyz]

See #507 (comment) - some endpoints like update_custom_reward have an enum as their return value:

twitch_api/src/helix/endpoints/points/update_custom_reward.rs

Lines 168 to 171 in c379795

	pub enum UpdateCustomReward {
	/// Reward updated
	Success(CustomReward),
	}

These should be removed (there are a few more - see linked PR and beyond(?)). Note that this is a breaking change.