Simplify BN_mod_{lshift1,sub}_quick().

Simplify BN_mod_{lshift1,sub}_quick().

The BN_mod_.*_quick() functions require that their inputs are non-negative
and are already reduced. As such, they can and should use BN_ucmp() and
BN_usub() instead of BN_cmp() and BN_add()/BN_sub() (which internally call
BN_uadd()/BN_usub() and potentially BN_cmp()).

ok tb@