Remove the RelaxAll overrule in MCAssembler::fixupNeedsRelaxation,

because that method is only getting called for MCInstFragment. These
fragments aren't even generated when RelaxAll is set, which is why the
flag reference here is superfluous. Removing it simplifies the code
with no harmful effects.

An assertion is added higher up to make sure this path is never
reached.

llvm-svn: 169886