registry: Remove from custom_type_dependencies when removing expr

In order to maintain the invariant that everything in the values of
`ExprRegistry::custom_type_dependencies` references a query that exists,
we need to remove queries from those values when removing expressions.

Release-Note-Core: Fixed an issue where ReadySet would crash when
  replicating an ALTER TYPE statement referencing a type that was
  mentioned in a column of a dropped table.
Fixes: ENG-2763
Change-Id: I7e425d32a2c887466814b0cc06a997b8d49fc361
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/4594
Tested-by: Buildkite CI
Reviewed-by: Nick Marino <nick@readyset.io>

readyset-psql/tests/fallback.rs

@@ -681,7 +681,6 @@ async fn rename_column_then_create_view() {
 
 #[tokio::test(flavor = "multi_thread")]
 #[serial]
-#[ignore = "ENG-2763 Test causes server panic and hangs due to known bug"]
 async fn alter_enum_after_drop() {
     readyset_tracing::init_test_logging();
 

readyset-server/src/controller/sql/registry.rs

@@ -512,6 +512,10 @@ impl ExprRegistry {
                 self.expressions.remove(dependency_id);
             }
         }
+        for deps in self.custom_type_dependencies.values_mut() {
+            deps.remove(&query_id);
+        }
+
         // If we have only removed a reused cache, there is nothing else to clean up because the
         // expression does not exist in the graph.
         if removed_reused_cache.any(|name| expr_aliases.contains(&name)) {