Don't declare "incoming_queue" in every file including channel.h

Found with my wacky symbol-usage-enforcer.

src/or/channel.h

@@ -18,7 +18,7 @@ typedef void (*channel_cell_handler_fn_ptr)(channel_t *, cell_t *);
 typedef void (*channel_var_cell_handler_fn_ptr)(channel_t *, var_cell_t *);
 
 struct cell_queue_entry_s;
-TOR_SIMPLEQ_HEAD(chan_cell_queue, cell_queue_entry_s) incoming_queue;
+TOR_SIMPLEQ_HEAD(chan_cell_queue, cell_queue_entry_s);
 typedef struct chan_cell_queue chan_cell_queue_t;
 
 /**