spi: atmel: Really drop atmel_spi.h

spi: atmel: Really drop atmel_spi.h

First try dropping this was with commit 37434db29be4 ("spi: atmel: Drop
atmel_spi.h") back in 2018 which was reverted not much later with commit
5270df283676 ("Revert "spi: atmel: Drop atmel_spi.h"").

Second try dropping this was in 2020 with commit beeb34ac0cc6 ("spi:
atmel: Drop atmel_spi.h"), but that only moved all the definitions into
the source file and did not remove the header file.

Currently all of the definitions in the header file are (still)
contained in the source file, and the header file is include nowhere.

Fixes: beeb34ac0cc6 ("spi: atmel: Drop atmel_spi.h")
Signed-off-by: Alexander Dahl <ada@thorsis.com>