Fix UITraitCollection build warnings: Round 1

[aokj4ck]

• Resolve 4 UITraitCollection warnings

traitCollectionDidChange

iOS 8.0–17.0 Deprecated
In Swift, use registerForTraitChanges(_:handler:) or registerForTraitChanges(_:target:action:) instead. In Objective-C, use registerForTraitChanges:withHandler: or registerForTraitChanges:withTarget:action: instead.
— https://developer.apple.com/documentation/uikit/uitraitenvironment/traitcollectiondidchange(_:)

ShareViewController

• Testing on iPhone 17 in the simulator with all available Environment Overrides I was not able to find any reason for the trait collection to invoke configurePublishButtonApperance()
  • Important: use the ShareActionExtension target and invoke it from an external app / Safari share sheet
• All the configuration in configurePublishButtonApperance() should be handled by UIImage + Asset support for switching among light/dark mode

HUDButton

Steps to test:

1. Open the timeline
2. Tap on any image attached to a post
   • At this time the (X) and (ALT) buttons font legibility weight should match your default settings (bold/regular)
3. Use the Xcode debugger's Environment Overrides > Accessibility > Bold Text toggle

Now the (X) and (ALT) buttons font legibility should always match the Bold Text toggle environment override value

AvatarButton

• Resolve trait collection warning in AvatarButton by removing listening to every trait change
• Previously used in 1cbc385
• Previously, in that commit, AvatarButton had background colors and other customization but the trait collection changes are no longer necessary to update background colors / theming because the AvatarButton is displayed in the same style across light and dark modes

MastodonSDK UIImage extensions

• Remove unused code

[aokj4ck]

After these fixes, 23 warnings will remain git grep 'super.traitCollectionDidChange' | wc -l