Remove unused con_get_next()

This function has unused for a long time since the commit
8f4b9ddaa4.

include/con.h

@@ -378,13 +378,6 @@ orientation_t con_orientation(Con *con);
  */
 Con *con_next_focused(Con *con);
 
-/**
- * Get the next/previous container in the specified orientation. This may
- * travel up until it finds a container with suitable orientation.
- *
- */
-Con *con_get_next(Con *con, char way, orientation_t orientation);
-
 /**
  * Returns the focused con inside this client, descending the tree as far as
  * possible. This comes in handy when attaching a con to a workspace at the