site: Remove pointless check from decrypt_msg0

It is not necessary to check whether we have a current key before
attempting to unpick and decrypt a MSG0.  If we don't,
transform->reverse will simply fail due to lacking a key.

This check needs to be removed, because in forthcoming patches we will
want to be able to try to decrypt the packet with other keys, at which
point the lack of a currently valid data transfer key will not be
relevant and this check will be harmful.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>