lint: uncomment redundant check for nicer organization

`vec_size(x)` appears to do the same check and emit the same message (since we
also use the name `x`), but adding back the check doesn't seem to have a
noticeable impact on `as_epi_archive()` or `revision_summary()` performance, and
if an error is raised, it's probably nicer for the trace to point to dedicated
validation code.

R/archive.R

@@ -377,7 +377,7 @@ removed_by_compactify <- function(df, keys, tolerance) {
 #' @importFrom vctrs vec_c vec_slice vec_size
 #' @export
 vec_position_lag <- function(x, n) {
-  # obj_check_vector(x)
+  obj_check_vector(x)
   assert_count(n)
   if (length(x) == 0L) {
     x