Unify DES library: ncbc_enc.c wasn't used, but its content was almost

duplicated in cbc_enc.c (without IV updating) and in des_enc.c

As pointed out by others on the openssl-dev list, des_cbc_encrypt (without
IV updating; defined in cbc_enc.c) exists only for historical reasons:
des_ncbc_encrypt should be used instead (and the caller does not have
to manually update the IV).

If des_cbc_enrypt is not needed for backwards compatibility, the
definition of des_ncbc_encrypt should be put back into des_enc.c, and
both cbc_enc.c and ncbc_enc.c can be deleted.

If des_cbc_encrypt *is* needed for backwards compatibility, its behaviour
obviously should not change (i.e., don't add IV updating).