do_last: kill a rudiment of old ->d_revalidate() workaround

There used to be time when ->d_revalidate() couldn't return an error.
So intents code had lookup_instantiate_filp() stash ERR_PTR(error)
in nd->intent.open.filp and had it checked after lookup_hash(), to
catch the otherwise silent failures.  That had been introduced by
commit 4af4c52f.  These days
->d_revalidate() can and does propagate errors back to callers
explicitly, so this check isn't needed anymore.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>