Validate matched containers for "kill" command correctly.

We now execute the validations when "kill" is executed even if match
criteria are used. This prevents users from killing workspace containers,
which instead kills all clients (as before when not using criteria).

fixes #1761

include/tree.h

@@ -56,12 +56,6 @@ bool level_down(void);
  */
 void tree_render(void);
 
-/**
- * Closes the current container using tree_close_internal().
- *
- */
-void tree_close_con(kill_window_t kill_window);
-
 /**
  * Changes focus in the given way (next/previous) and given orientation
  * (horizontal/vertical).