st/mesa: fix weird UCMP opcode use for bool ubo load

st/mesa: fix weird UCMP opcode use for bool ubo load

I don't know what this code was trying to do but whatever it was it couldn't
have worked since negation of integer boolean inputs while not specified as
outright illegal (not yet at least) won't do anything since it doesn't affect
the result of comparison with zero at all. In fact it looks like the whole
instruction can just be omitted.

Reviewed-by: Marek Olšák <maraeo@gmail.com>