You remove a software interrupt handler by passing an ioexttd to the
device with a pointer to an Interrupt structure set in io_Data, the length
of the structure set in io_Length and td_remchangeint set in io_command.
You must pass it the same Interrupt structure used to add the handler.

    DiskIO->iotd_Req.io_Length = sizeof(struct Interrupt)
    DiskIO->iotd_Req.io_Data   = (APTR)Disk_Interrupt;
    DiskIO->iotd_Req.io_Command = TD_REMCHANGEINT;
    DoIO((struct IORequest *)DiskIO);

   Don't use with pre-V36 and earlier versions.
   --------------------------------------------
   Under pre-V36 and earlier versions of the Amiga system software,
   td_remchangeint does not work and should not be used.  instead, use
   the workaround listed in the "trackdisk.doc" of the Amiga ROM
   Kernel Reference Manual: Includes and Autodocs.