Stop using wk::validate_wk_wkt() and wk::validate_wk_wkb() with unclassed input

[paleolimbot]

In the forthcoming wk release, these were slated to change to error for objects that don't inherit from wk_wkt and wk_wkb, respectively. In s2, they're used to give pretty error messages for invalid WKB in s2::s2_geog_from_text()/wkb. I reverted the change to avoid breaking anything on CRAN, but this should get fixed in the development version. The correct usage would be attributes(x) <- NULL; wk::validate_wk_wkt(new_wk_wkt(x)).

[paleolimbot]

This was fixed in #155.