dt-bindings: mtd: Remove useless file about partitions

There is already a real partitions.yaml file, so assuming everybody
knows hot to read yaml schema now, this text file is no longer needed,
so drop it.

Depending on the situation, the lines referring to this file are either
dropped or edited to point to mtd.yaml which includes partition{,s}.yaml.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-3-miquel.raynal@bootlin.com