Commit 956b200a ("spi: spidev: Warn loudly if instantiated from DT
as "spidev"") states that there should not be spidev nodes in DTs.
Remove this non-HW description. There won't be a regression because it
won't bind since 2015 anyhow.

Fixes: 16e3e44c ("ARM: dts: stm32: Add support for the emtrion emSBC-Argon")
Cc: Reinhold Mueller <reinhold.mueller@emtrion.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>