fix: grouped_epi_archives are not epi_archives

We don't want to try to use an `epi_archive` method implementation on a
`grouped_epi_archive`, or have `is_epi_archive` succeed on them even with
`grouped_okay = FALSE`, to prevent attempted extraction of nonexistent fields.

R/grouped_epi_archive.R

@@ -80,7 +80,7 @@ new_grouped_epi_archive <- function(x, vars, drop) {
     list(
       private = private
     ),
-    class = c("grouped_epi_archive", "epi_archive")
+    class = c("grouped_epi_archive")
   ))
 }