spi: fsl: Remove unused extern declarations

commit b36ece83 ("spi/mpc8xxx: refactor the common code for SPI/eSPI controller")
left mpc8xxx_spi_bufs() behind.
And since commit 3c5395b6 ("spi: fsl-(e)spi: simplify cleanup code")
mpc8xxx_spi_remove() is not used anymore.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20230725135411.21152-1-yuehaibing@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>