iio: gyro: mpu3050: Drop unused symbol exports.

This driver is built as a single module.  There is an note in the makefile
about SPI support being a reason to split the build in future. That
support has never arrived so drop these symbol exports.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220220175149.503495-2-jic23@kernel.org