iio: STMicroelectronics: remove three useless selects

Drivers for STMicroelectronics accelerometers, gyroscopes, and
magnetometers were added in v3.9. They all have a (similar) select
statement in their Kconfig files for a non-existant Kconfig symbol.
These select statements can safely be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>