mtd: spi-nor: core: Remove reference to spi-nor.c

Remove the references to the old spi-nor.c file.
The old drivers/mtd/spi-nor/spi-nor.c file
is not more present and now some of its code is contained in:
drivers/mtd/spi-nor/core.c

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
[tudor.ambarus@microchip.com:
- remove change in Documentation/driver-api/mtd/spi-nor.rst.
The documentation has to be rewritten entirely.
- update commit message]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20210126092516.1431913-1-f.suligoi@asem.it