Use the default focused tab closing behavior

Currently when closing the currently focused slot, the next slot to gain focus is the earliest created. This is because the default behavior, which is to open the next tab at the same position or the new last tab if the previous tab was closed, used in other gnome apps like epiphany and terminal is overridden by manually setting active slot to earliest inactive one.

To provide consistent behavior, remove code that overwrites the default gtk notebook tab closing behavior.

Fixes #1567 (closed)