rest_flag is allowed in build-namelist, but not in the FORTRAN namelist

### Brief summary of bug

The namelist item "rest_flag" to turn off restart handling is allowed in build-namelist at the preview_namelist level -- but not in the Fortran code. So using it is allowed at preview_namelist, but it dies at runtime.

### General bug information

**CTSM version you are using:** ctsm5.3.061-6-ga27de36fe
**Does this bug cause significantly incorrect results in the model's science?** No

**Configurations affected:** Only when you set rest_flag in user_nl_clm

### Details of bug

It's only defined in namelist definition as follows...

``` xml

If FALSE, don't write any restart files.

```

From git blame, I added this back in 2008-2010. I am not sure why.

### Important details of your setup / configuration so we can reproduce the bug

It dies with the usual namelist issue in controlMod.F90:

```
Attempting to initialize run control settings .....
Read in clm_inparm namelist from: lnd_in
ENDRUN:
ERROR reading clm_inparm namelistERROR in /glade/work/erik/ctsm_worktrees/cleanupwork/src/main/controlMod.F90 at line 377
```