Revert "mtd: core: always create master device"

The idea behind this patch was to always let a "master" mtd device
available to anchor runtime PM. Historically, there was no mtd device
representing the whole storage as soon as partitions were coming into
play. The introduction of CONFIG_MTD_PARTITIONED_MASTER allowed to keep
this "master" device, but was not enabled by default to avoid breaking
existing users (otherwise the mtd device numbering would be totally
messed up with an off by 1, at least).

The approach of adding an mtd_master class on top of partitioned mtd
devices is breaking the mtd core in many creative ways, so better think
again this approach and revert the faulty changes for now.

This reverts commit 0aa7b390.

Fixes: 0aa7b390 ("mtd: core: always create master device")
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>