feat(preferences): remove options

[GeopJr]

Following options are unpopular or a default value is clearly the preferred one. This PR will remove them.

If you use any of these, I'm open to leaving it in gsettings without a UI toggle so speak now or forever hold your peace

Removed Option	Default	Comments
color scheme	following system theme	-
show preview card	true	-
scale emoji on hover	true	-
media viewer expand pictures	true	-
use blurhash	true	-

[LukaszH77]

I use "media viewer expand pictures"

[GeopJr]

Would you mind if it was gsettings only or do you think it should stay as a switch in the GUI?

edit: (by gsettings only I mean you can change it with dconf-editor or the gsettings cli tool)

[LukaszH77]

I'm fine with using gsettings to turn that off.