EAMxx: remove the file user_nl_scream as soon as possible

[bartgol]

EAMxx does not use namelist files for user defined options, relying instead on explicit atmchange calls. Yet, CIME generates user_nl_<COMPNAME> for all components. The content of user_nl_scream is

$ cat user_nl_scream 
!----------------------------------------------------------------------------------
! Users should add all user specific namelist changes after these comments
! in the form of
!   namelist_var = new_namelist_value
! *** EXCEPT FOR ***
! 1. DO NOT CHANGE config_start_time, config_run_duration, config_stop_time,
!    config_do_restart, config_Restart_timestamp_filename, config_calendar_type,
!    config_set_restingThickness_to_IC, config_alter_ICs_for_pbcs
! 2. To preview the namelists, invoke $CASEROOT preview-namelists and look at
!    $RUNDIR/atm_in
!----------------------------------------------------------------------------------

which is also a bit more confusing since it mentions $RUNDIR/atm_in, which does not exist (EAMxx uses $RUNDIR/data/scream_input.yaml, which is generated by EAMxx's buildnml).

Luckily, @jgfouca found out that this file is created before the call to buildnml. So EAMxx could rm this file inside buildnml, avoiding any confusion for the user, which will never see this file once ./case.setup has completed.