timer-face: Remove "Select Duration Title"

Revival of !242 (closed) (I deleted my fork and forgot that this MR was open).

This title isn't present anymore acc. to the latest mockups: https://gitlab.gnome.org/Teams/Design/app-mockups/-/blob/master/clocks/clocks.png

BTW, the changes in timer-face.vala weren't necessary like in !242 (closed), but thanks to @AlbertoHSande again!