docs: mips: remove no longer needed au1xxx_ide.rst documentation

Since the au1xxx-ide IDE host driver is no longer needed its documentation
can be removed.

Acked-by: Manuel Lauss <manuel.lauss@gmail.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>