Revert "mtd: spi-nor: Add capability to disable flash quad mode"

As the only user has been removed in previous patch, let's revert
this one together.

This reverts commit be192209.

Reported-by: Matthias Weisser <m.weisser.m@gmail.com>
Suggested-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/1599205640-26690-2-git-send-email-yangyicong@hisilicon.com