gcrypt: Remove open-coded OAEP padding.

gcrypt: Remove open-coded OAEP padding.

The open-coded OAEP padding implementaton of gcrypt.c was necessary to
support old libgcrypt versions which do not have the oaep flag. These
days, everybody ought to have moved on to 1.5.0 (released 2011)
or later, so lets require this version from now on.

decode_rsa() used its key_size parameter only for old gcrypt versions,
so the parameter can be removed. Also, the E_MPI_PRINT error code
has become unused and can be removed from error.h.

Since rsa_decrypt_sexp is now constant, there is no need to have a
variable for this, so we can make it a preprocessor definition instead.