Simplify pending builds migration and db structure

Applied 0001-Minor-tweaks-to-ci_pending_builds.patch from @abrandl.

db/migrate/20210512120122_add_pending_builds_table.rb

@@ -2,9 +2,9 @@
 
 class AddPendingBuildsTable < ActiveRecord::Migration[6.0]
   def up
-    create_table :ci_pending_builds, id: false, if_not_exists: true do |t|
+    create_table :ci_pending_builds, id: false do |t|
       t.timestamps_with_timezone null: false
-      t.references :build, primary_key: true, index: { unique: true }, foreign_key: { to_table: :ci_builds, on_delete: :cascade }
+      t.references :build, primary_key: true, default: nil, index: false, foreign_key: { to_table: :ci_builds, on_delete: :cascade }
       t.references :project, index: true, null: false, foreign_key: { on_delete: :cascade }
     end
   end

db/structure.sql

@@ -10759,15 +10759,6 @@ CREATE TABLE ci_pending_builds (
     project_id bigint NOT NULL
 );
 
-CREATE SEQUENCE ci_pending_builds_build_id_seq
-    START WITH 1
-    INCREMENT BY 1
-    NO MINVALUE
-    NO MAXVALUE
-    CACHE 1;
-
-ALTER SEQUENCE ci_pending_builds_build_id_seq OWNED BY ci_pending_builds.build_id;
-
 CREATE TABLE ci_pipeline_artifacts (
     id bigint NOT NULL,
     created_at timestamp with time zone NOT NULL,
@@ -19484,8 +19475,6 @@ ALTER TABLE ONLY ci_job_variables ALTER COLUMN id SET DEFAULT nextval('ci_job_va
 
 ALTER TABLE ONLY ci_namespace_monthly_usages ALTER COLUMN id SET DEFAULT nextval('ci_namespace_monthly_usages_id_seq'::regclass);
 
-ALTER TABLE ONLY ci_pending_builds ALTER COLUMN build_id SET DEFAULT nextval('ci_pending_builds_build_id_seq'::regclass);
-
 ALTER TABLE ONLY ci_pipeline_artifacts ALTER COLUMN id SET DEFAULT nextval('ci_pipeline_artifacts_id_seq'::regclass);
 
 ALTER TABLE ONLY ci_pipeline_chat_data ALTER COLUMN id SET DEFAULT nextval('ci_pipeline_chat_data_id_seq'::regclass);
@@ -22546,8 +22535,6 @@ CREATE UNIQUE INDEX index_ci_job_variables_on_key_and_job_id ON ci_job_variables
 
 CREATE UNIQUE INDEX index_ci_namespace_monthly_usages_on_namespace_id_and_date ON ci_namespace_monthly_usages USING btree (namespace_id, date);
 
-CREATE UNIQUE INDEX index_ci_pending_builds_on_build_id ON ci_pending_builds USING btree (build_id);
-
 CREATE INDEX index_ci_pending_builds_on_project_id ON ci_pending_builds USING btree (project_id);
 
 CREATE INDEX index_ci_pipeline_artifacts_failed_verification ON ci_pipeline_artifacts USING btree (verification_retry_at NULLS FIRST) WHERE (verification_state = 3);