samples: drivers: adc: adc_sequence: remove stm32f3 from allow list

The stm32f3_disco board requires an external AD4114 sensor for the
ADC test to pass. Since this sensor is not available by default in
CI environments, the test fails.
Removing stm32f3 from the allow list prevents false negatives during
automated testing.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>