z80: split the IDE driver __DISCARD up and remove the feature

z80: split the IDE driver __DISCARD up and remove the feature

We can't have multiple banked and unbanked segments in one object file as
we then have no way to make the banked linker and binmunge fix them up. The
common piece is ok as it doesn't call out to other segments in the same file
likewise calls to common code is ok. Calls betweem banks however don't end
well.