boards nrf54l15bsim: Do not work around peripheral clock issue

In 923d313a the clock frequency in DTS
for the UART00 was fixed, but not for the simulated target. This was
likely due to the HW models modeling it as 16MHz instead of 128MHz for
this particular one as it is in reality.

Now that the HW models have been fixed, let's let this clock be
configured like for real HW.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>