cipher: Deprecate GCRY_PK_ELG_E.

* cipher/elgamal.c (_gcry_pubkey_spec_elg_e): Remove.
* cipher/pubkey.c (pubkey_list): Remove double included
_gcry_pubkey_spec_elg.
(map_algo): MAke ELG_E to ELG.

It does not make sense to have different algorithm identifiers for
Elgamal encryption and the obsolete Elgamal signing. Thus mark one of
them as deprecated and remove the useless extra spec info in
elgamal.c. Frankly it is questionable why at all we need the
algorithm identifiers; they are a remain from the time when Libgcrypt
was part of GnuPG. With the s-expression based API the IDs they don't
make much sense.

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