docs: ABI: testing: iio: stm32: remove re-introduced unsupported ABI

Remove unsupported ABI that has been re-introduced due to a rebase hunk.
This ABI has been moved in the past in commit b299d004
("IIO: stm32: Remove quadrature related functions from trigger driver")

This also fixes a couple of warnings seen with:
./scripts/get_abi.pl validate 2>&1|grep iio

Fixes: 34433332

 ("docs: ABI: testing: make the files compatible with ReST output")
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Link: https://lore.kernel.org/r/1604685016-2434-1-git-send-email-fabrice.gasnier@st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>