IIO: stm32: Remove quadrature related functions from trigger driver

Quadrature feature is now hosted on it own framework.
Remove quadrature related code from stm32-trigger driver to avoid
code duplication and simplify the ABI.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>