mtd: spi-nor: stm32: remove the driver as it was replaced by spi-stm32-qspi.c

There's a new driver using the SPI memory interface of the
SPI framework at spi/spi-stm32-qspi.c, which can be used
together with m25p80.c to replace the functionality of
this SPI NOR driver.

The "new" driver uses the same dt properties and not affects
the legacy compatibility.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>