move gaps-specific logic out of con.c and render.c into gaps.c

include/con.h

@@ -525,13 +525,6 @@ void con_set_urgency(Con *con, bool urgent);
  */
 char *con_get_tree_representation(Con *con);
 
-/**
-  * Calculates the effective gap sizes for a container depending
-  * on whether it is adjacent to the edge of the screen or another
-  * container.
-  */
-gaps_t calculate_effective_gaps(Con *con);
-
 /**
  * force parent split containers to be redrawn
  *