build: Stop building a separate libgnome-shell-base

The static library was split out to avoid duplicate compilation
of mutter-independent libgnome-shell sources when we had a
wayland/non-wayland split. With the separate libmutter-wayland
library long gone, there's no reason to make the build more
complicated than it needs to be, so fold libgnome-shell-base
back into the regular libgnome-shell.

https://bugzilla.gnome.org/show_bug.cgi?id=783206