relay: Remove max middle cells dead code

Next commit is addressing the circuit queue cell limit so cleanup before doing
anything else.

Part of #25226

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/or/or.h

@@ -3524,12 +3524,6 @@ typedef struct or_circuit_t {
    * exit-ward queues of this circuit; reset every time when writing
    * buffer stats to disk. */
   uint64_t total_cell_waiting_time;
-
-  /** Maximum cell queue size for a middle relay; this is stored per circuit
-   * so append_cell_to_circuit_queue() can adjust it if it changes.  If set
-   * to zero, it is initialized to the default value.
-   */
-  uint32_t max_middle_cells;
 } or_circuit_t;
 
 #if REND_COOKIE_LEN != DIGEST_LEN

src/or/relay.c

@@ -99,9 +99,6 @@ static void adjust_exit_policy_from_exitpolicy_failure(origin_circuit_t *circ,
                                                   entry_connection_t *conn,
                                                   node_t *node,
                                                   const tor_addr_t *addr);
-#if 0
-static int get_max_middle_cells(void);
-#endif
 
 /** Stop reading on edge connections when we have this many cells
  * waiting on the appropriate queue. */