Remove unnecessary index for `vulnerabilities` table on `project_id`

db/migrate/20201202142751_drop_index_vulnerabilities_on_project_id.rb

+# frozen_string_literal: true
+
+class DropIndexVulnerabilitiesOnProjectId < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_vulnerabilities_on_project_id'
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name :vulnerabilities, INDEX_NAME
+  end
+
+  def down
+    add_concurrent_index :vulnerabilities, :project_id, name: INDEX_NAME
+  end
+end

db/schema_migrations/20201202142751

+cb11dc9996b1706feaa8a53f96cbaa6209a4d07b3be9e88ebc3d1e1ada561287
\ No newline at end of file

db/structure.sql

@@ -22470,8 +22470,6 @@ CREATE INDEX index_vulnerabilities_on_last_edited_by_id ON vulnerabilities USING
 
 CREATE INDEX index_vulnerabilities_on_milestone_id ON vulnerabilities USING btree (milestone_id);
 
-CREATE INDEX index_vulnerabilities_on_project_id ON vulnerabilities USING btree (project_id);
-
 CREATE INDEX index_vulnerabilities_on_project_id_and_state_and_severity ON vulnerabilities USING btree (project_id, state, severity);
 
 CREATE INDEX index_vulnerabilities_on_resolved_by_id ON vulnerabilities USING btree (resolved_by_id);