fix(DENG-11237): drop schema.yaml + destination_table:null for GCS export

The prior fix used scheduling.destination_table: null, but that field with a
query.py trips DAG task validation (task.py: destination_table None and
query_file_path None -> 'One of destination_table or query_file_path must be
specified'); the only existing destination_table:null tasks are query.sql.

Use the reviewer's primary suggestion instead: ship only metadata.yaml +
query.py (no schema.yaml), matching the newtab_merino GCS-export tasks. On the
current deploy code a missing schema is a non-fatal SkippedDeployException
(dry-run path) / _resolve_isolated_schema is skipped when there's no manifest,
so no empty table is created and stage-deploy doesn't fail. CSV column contract
stays documented in metadata.yaml.

Author: phil-lee70

sql/moz-fx-data-shared-prod/firefox_referral_derived/referral_installs_totals_to_gcs_v1/metadata.yaml

@@ -10,10 +10,9 @@ description: |-
   One file per run date named `referral_data-<date>.csv` (date = Airflow {{ds}}),
   so an Airflow retry overwrites the same object. Runs daily.
 
-  This task only extracts to GCS; it does not build a queryable BigQuery table
-  (scheduling.destination_table is null so deploy skips table creation). The
-  sibling schema.yaml documents the CSV columns and lets stage-deploy resolve
-  the query.py artifact.
+  This task only extracts to GCS; it does not build a queryable BigQuery table.
+  Like the newtab_merino GCS-export tasks, it ships only metadata.yaml + query.py
+  (no schema.yaml), so deploy skips table creation (missing-schema skip).
 
   Destination bucket `fx-referral-data-prod` (project moz-fx-springfield-prod) is
   owned by the Website team; the Airflow workload service account
@@ -26,7 +25,6 @@ labels:
   owner1: phlee
 scheduling:
   dag_name: bqetl_firefox_referral
-  destination_table: null
   arguments:
   - --source-project=moz-fx-data-shared-prod
   - --source-dataset=firefox_referral_derived