Remove superfluous ERRP_GUARD()

Macro ERRP_GUARD() is only needed when we want to dereference @errp or
pass it to error_prepend() or error_append_hint().  Delete superfluous
ones.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-15-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>