cmux: Remove round-robin circuit policy

Since 0.2.4, tor uses EWMA circuit policy to prioritize. The previous
algorithm, round-robin, hasn't been used since then but was still used as a
fallback.

Now that EWMA is mandatory, remove that code entirely and enforce a cmux
policy to be set.

This is part of a circuitmux cleanup to improve performance and reduce
complexity in the code. We'll be able to address future optimization with this
work.

Closes #25268

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

src/test/test_channel.c

@@ -281,6 +281,7 @@ new_fake_channel(void)
   chan->state = CHANNEL_STATE_OPEN;
 
   chan->cmux = circuitmux_alloc();
+  circuitmux_set_policy(chan->cmux, &ewma_policy);
 
   return chan;
 }
@@ -582,8 +583,6 @@ test_channel_outbound_cell(void *arg)
    * to the channel's cmux as well. */
   circuit_set_n_circid_chan(TO_CIRCUIT(circ), 42, chan);
   tt_int_op(channel_num_circuits(chan), OP_EQ, 1);
-  tt_assert(!TO_CIRCUIT(circ)->next_active_on_n_chan);
-  tt_assert(!TO_CIRCUIT(circ)->prev_active_on_n_chan);
   /* Test the cmux state. */
   tt_ptr_op(TO_CIRCUIT(circ)->n_mux, OP_EQ, chan->cmux);
   tt_int_op(circuitmux_is_circuit_attached(chan->cmux, TO_CIRCUIT(circ)),

src/test/test_circuitmux.c

@@ -7,6 +7,7 @@
 #include "or.h"
 #include "channel.h"
 #include "circuitmux.h"
+#include "circuitmux_ewma.h"
 #include "relay.h"
 #include "scheduler.h"
 #include "test.h"
@@ -45,6 +46,7 @@ test_cmux_destroy_cell_queue(void *arg)
   cmux = circuitmux_alloc();
   tt_assert(cmux);
   ch = new_fake_channel();
+  circuitmux_set_policy(cmux, &ewma_policy);
   ch->has_queued_writes = has_queued_writes;
   ch->wide_circ_ids = 1;