refactor(core): drop ayatana-tray and gtk-tray Cargo features

[lucasfernog]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

[FabianLars]

• Since we're talking about .deb here (and therefore mainly about debian & ubuntu) we should probably use ayatana as the default.
• iirc, we now depend on pkg-config at runtime, so we need to add that to the Depends field too right?

[lucasfernog]

Yeah you're right, I'll make that change.

[lucasfernog]

Ref #3859