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.

src/con.c

@@ -286,14 +286,14 @@ void con_close(Con *con, kill_window_t kill_window) {
         for (child = TAILQ_FIRST(&(con->focus_head)); child;) {
             nextchild = TAILQ_NEXT(child, focused);
             DLOG("killing child = %p.\n", child);
-            tree_close_internal(child, kill_window, false, false);
+            tree_close_internal(child, kill_window, false);
             child = nextchild;
         }
 
         return;
     }
 
-    tree_close_internal(con, kill_window, false, false);
+    tree_close_internal(con, kill_window, false);
 }
 
 /*
@@ -1437,6 +1437,9 @@ Con *con_next_focused(Con *con) {
         DLOG("selecting workspace for dock client\n");
         return con_descend_focused(output_get_content(con->parent->parent));
     }
+    if (con_is_floating(con)) {
+        con = con->parent;
+    }
 
     /* if 'con' is not the first entry in the focus stack, use the first one as
      * it’s currently focused already */
@@ -1955,7 +1958,7 @@ static void con_on_remove_child(Con *con) {
         if (TAILQ_EMPTY(&(con->focus_head)) && !workspace_is_visible(con)) {
             LOG("Closing old workspace (%p / %s), it is empty\n", con, con->name);
             yajl_gen gen = ipc_marshal_workspace_event("empty", con, NULL);
-            tree_close_internal(con, DONT_KILL_WINDOW, false, false);
+            tree_close_internal(con, DONT_KILL_WINDOW, false);
 
             const unsigned char *payload;
             ylength length;
@@ -1976,7 +1979,7 @@ static void con_on_remove_child(Con *con) {
     int children = con_num_children(con);
     if (children == 0) {
         DLOG("Container empty, closing\n");
-        tree_close_internal(con, DONT_KILL_WINDOW, false, false);
+        tree_close_internal(con, DONT_KILL_WINDOW, false);
         return;
     }
 }