chore(devtools): delete the dag_export bin

Its export path ran the legacy PhysicalPlanCostModel, which Stage 3 does
not use; stage_pipeline replaces it for inspection. The lost
evidence-changes-selection coverage is tracked in #615.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: zzylol

crates/devtools/Cargo.toml

@@ -16,7 +16,7 @@ asap-logical-optimizer = { path = "../logical-optimizer" }
 # plans with. A devtool, not a stage crate, so the #572 guards allow it.
 asap-executor = { path = "../executor" }
 
-# Used by the show_ir / dag_export / variant_coverage bins (catalog schemas,
+# Used by the show_*_ir / stage_pipeline / variant_coverage bins (catalog schemas,
 # async SQL path, JSON output) and by the topk_ir / canonical_examples
 # examples. Regular deps, not dev-deps: `[[bin]]` targets can't see dev-deps.
 asap-types = { path = "../types" }