crypto/ec/curve25519.c: remove redundant fe[51]_cswap.

3 least significant bits of the input scalar are explicitly cleared,
hence swap variable has fixed value [of zero] upon exit from the loop.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/5408)