Simplify tree_close_internal

Simplify tree_close_internal

This commit makes multiple changes in tree_close_internal. I didn't
split them because they are not completely independent.

- Remove force_set_focus parameter
This parameter was always set to `false` throughout the code base except
for one case where it was set to `(con == focused)`, when killing a
floating con's parent (the one with type CT_FLOATING_CON). But this case
is not needed anymore since the special handling of CT_FLOATING_CONs in
con_next_focused was removed in #2941.
- Assume that con_next_focused does not returned a container of type
CT_DOCKAREA. This is reasonable since con_next_focused uses the
focus_head stack and has special handling of CT_DOCKAREA containers.
- Remove is_mapped
This variable was only used in the if block towards the end of
tree_close_internal. Ignoring the, now removed, dockarea code and the
use of force_set_focus this block performed only one useful action:
focus the `next` container when `con == focused`. `con == focused` was a
necessary and sufficient condition for the con_activate call:
if `con != focused` we could reach the inner if blocks because of the
other conditions but would never focus another container. If `con ==
focused` then all other conditions would be irrelevant.
- Remove special handling of floating containers
Since the `next` focused container is calculated through the parent for
floating containers, I moved this code to con_next_focused.
Also, because of the removal of force_set_focus, it appears that we can
call con_on_remove_child for floating containers as well.