api: Remove arbitrary restriction from gpgme_op_verify.

* src/verify.c (verify_start): Do not return GPG_ERR_INV_VALUES when
when SIGNED_TEXT is not given.

The original idea behind this restriction probably was that it is
useless to verify a non-detached signatures without also returning the
signed text. However, it is sometimes useful to just check the
signature, for example to see who signed it.

• Signed-off-by: Werner Koch <wk@gnupg.org>