boards: arm: disco_l475_iot1: remove MCU slow down when using BlueRNG-MS

Remove unnecessary MCU slow down to 16Mhz when using BLE chip BlueNRG-MS

Signed-off-by: Yaël Boutreux <yael.boutreux@st.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>