LibWeb: Resolve FIXME around shorthand properties in remove_property()

[Calme1709]

This exposes some false-positive sub-tests in the font-computed.html test which are now correctly marked as failed.

Resolves an issue identified in 94f5a51

[tcl3]

This appears to be doing the same thing as #4902. I think mine is probably doing some unnecessary work, but I'll defer to someone who knows more about CSS to provide some more useful feedback.