6in4: send password unhashed

The preferred he.net /nic/update endpoint expects the password or updatekey in
plain text and not as md5 sum, therfore remove the hashing operation from the
script.

This effectively renders the "updatekey" option redundant but we keep it around
for backwards compatibility. Both "option password" and "option updatekey" will
have end up in the "&password=" parameter of the update url and are passed through
unmodified.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 41358