boards: particle_boron: Fix init of modem pins

The Boron has a power switch that controls power to a level shifter
for the modem's serial line. Previously, this switch was being enabled
after the modem driver had already started. It is now controlled using
a fixed regulator.

The board initialization was also configuring the V_INT_DETECT pin.
However, this pin is already handled as mdm-vint-gpios in the dts when
the ublox-sara-r4 driver is used. Since it is not used otherwise,
there is no need to configure it in the board init.

Fixes: #56883
Signed-off-by: Göran Weinholt <goran@weinholt.se>