Remove special handling of floating containers in con_next_focused

Remove special handling of floating containers in con_next_focused

Explanation for the changed test:
After $third is switched to floating, the test moves focus to $second.
So, the parent of $second (the stacked container) is above $third in the
focus stack and it's children ($first, $second) should get focused
before $second. When $second is switched to floating the correct focus
order for the workspace should be $second->parent (floating con is the
parent) > $first->parent (stacked con) > $third.

Fixes #1975