iio: accel: Remove unused enum in st_accel

The st_accel_type enum is not used anywhere else in the code, and can be
removed

Suggested-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Signed-off-by: Sicelo A. Mhlongo <absicsz@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220307132502.73854-3-absicsz@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>