Revert: Implement SI register DMA busy bit

The implementation of the SI regiser DMA busy bit caused regressions in games
like Mischief Makers (no gamepad input is detected by the game). The only known
benefit seems to be the reduced message spam about duplicated interrupts.

The message spam caused by some ROMS can be tolerated until the cause of the
regression is better understood.

This reverts 734b67b8ff647cc355b5f6b781799c42e75e81ea ("Implement SI register
DMA busy bit")