Eliminate once-used check_error utility function

include/util.h

@@ -98,14 +98,6 @@ bool update_if_necessary(uint32_t *destination, const uint32_t new_value);
  */
 void exec_i3_utility(char *name, char *argv[]);
 
-/**
- * Checks a generic cookie for errors and quits with the given message if
- * there was an error.
- *
- */
-void check_error(xcb_connection_t *conn, xcb_void_cookie_t cookie,
-                 char *err_message);
-
 /**
  * Checks if the given path exists by calling stat().
  *