boards: arm: disco_l475_iot1 do not declare adc1 input on PA0

Do not declare the in5 on pin PA0 of the adc1 since it makes
conflict with uart4_tx_pa0.
Other ADC1 adc1_in1_pc0 or adc1_in2_pc1 are not available
(used by the i2c3).

Signed-off-by: Francois Ramu <francois.ramu@st.com>