compute: drop migration-plan labels from temporal-bucketing comments

The comments named the migration plan's nodes, which mean nothing to a reader,
and described the union input in terms of a mixed-variant `concat_many` case
rather than what the code does.

Author: antiguru

src/compute/src/render.rs

@@ -1378,10 +1378,9 @@ impl<'scope, T: RenderTimestamp + MaybeBucketByTime> Context<'scope, T> {
                             .get(&self.config_set)
                             .try_into()
                             .expect("must fit");
-                        // Temporal bucketing (node C8) is `Vec`-internal: decode
-                        // the edge into it, then re-encode the `Vec` result to
-                        // columnar so this Union input stays columnar and
-                        // `concat_many` sees no mixed variants.
+                        // Temporal bucketing operates on `Vec`: decode the edge
+                        // into it, then re-encode the result so this Union input
+                        // is a columnar edge like every other.
                         let os = os.into_vec();
                         CollectionEdge::Columnar(vec_to_columnar(
                             T::maybe_apply_temporal_bucketing(

test/sqllogictest/temporal_bucketing.slt

@@ -278,12 +278,12 @@ Target cluster: quickstart
 EOF
 
 # -----------------------------------------------------------------------------
-# Runtime tests (node Ta). With `enable_compute_temporal_bucketing` on, the
+# Runtime tests. With `enable_compute_temporal_bucketing` on, the
 # bucketed dataflow edges are re-encoded to the columnar representation instead
 # of re-wrapping `Vec`. These tests turn the flag on and assert the bucketed
 # dataflows still produce the correct logical results, and that a consolidating
 # `Union` concatenating a bucketed input with a `Direct` input yields the right
-# output (the mixed `concat_many` case, now columnar on both legs).
+# output, which reaches `concat_many` as a columnar edge like the other leg.
 # -----------------------------------------------------------------------------
 
 simple conn=mz_system,user=mz_system
@@ -344,7 +344,7 @@ SELECT * FROM rt_topk
 
 # Mixed Union: `EXCEPT ALL` of a temporal-filtered leg (bucketed) against a
 # plain relation (Direct) lowers to a consolidating `Union` with per-input
-# strategies `[TemporalBucketing, Direct]`. After node Ta both legs reach
+# strategies `[TemporalBucketing, Direct]`. Both legs reach
 # `concat_many` as columnar edges.
 query T multiline
 EXPLAIN PHYSICAL PLAN AS VERBOSE TEXT FOR