chore: revert feat(enterprise/coderd): allow system users to be added to groups (#19254)

This reverts commit b200fc8e67
(https://github.com/coder/coder/pull/18341).

coderd/database/queries/organizationmembers.sql

@@ -89,8 +89,6 @@ WHERE
 			organization_id = @organization_id
 		ELSE true
 	END
-  -- Filter by system type
-	AND CASE WHEN @include_system::bool THEN TRUE ELSE is_system = false END
 ORDER BY
 	-- Deterministic and consistent ordering of all users. This is to ensure consistent pagination.
 	LOWER(username) ASC OFFSET @offset_opt