Kill user multitheme support or make it a setting

Now that Garland *and* Seven is enabled in core, we have this interesting situation, where admin users will always get a theme selector screen. (Note that Dries forgot to commit the theme screenshot, see: #538646: Screenshot missing from Seven theme). This is bad. People should not be able to select the admin theme as a site theme, if they are configured separate. So we can do a few things:

(A) Kill the per-user theme support in core and make it contrib
(B) Make "this theme can be user selected" a setting somehow - gonna be a ugly and complex for the admin but would let admins use different themes for site sections (think themekey module, sections module, etc), while not distracting admins with this screen
(C) Just not display the admin in this form when it is set explicitly different to the site theme
(D) Just live with the fact. Only user #1 sees this at all times, other admins can have this permission revoked to be able to select themes (but in the admin role it will be granted automatically).
(E) Whatever else?

I'm not sure whether (A) or (C) is the best. I'd not add another magic setting for this.