schema/pgsql: Drop checkcommand_argument.argument_key storage mode

During #792, the storage mode for this column got lost. After further
inspection, setting a distinct storage mode for a column of type varchar
seems unnecessary.

schema/pgsql/schema.sql

@@ -855,7 +855,6 @@ CREATE TABLE checkcommand_argument (
 ALTER TABLE checkcommand_argument ALTER COLUMN id SET STORAGE PLAIN;
 ALTER TABLE checkcommand_argument ALTER COLUMN environment_id SET STORAGE PLAIN;
 ALTER TABLE checkcommand_argument ALTER COLUMN checkcommand_id SET STORAGE PLAIN;
-ALTER TABLE checkcommand_argument ALTER COLUMN argument_key SET STORAGE PLAIN;
 ALTER TABLE checkcommand_argument ALTER COLUMN properties_checksum SET STORAGE PLAIN;
 
 COMMENT ON COLUMN checkcommand_argument.id IS 'sha1(environment.id + checkcommand_id + argument_key)';