[MC] Delete an overload of MCExpr::evaluateKnownAbsolute and its associated hack

The hack dated back to 2010 (r121076) and was documented by r122144:

  // FIXME: The use if InSet = Addrs is a hack. Setting InSet causes us
  // absolutize differences across sections and that is what the MachO writer
  // uses Addrs for.

llvm-svn: 369337