AUDIO: Do not add extrapath to SearchMan in MT-32 emu code.

At the point where the emulator is created extrapath should already been
added to extrapath. If not, the check in checkDevice already failed and thus
adding it would be too late anyway.

It seems this was added in 805b21181ab7138da6960ade703b25716120fc29. The
comment about it being a HACK has been removed in bbad3f333a9227ccb1de633a0fe92d9e01ad7bb3
but it's not clear to my why... At any rate, this should not be here.