build-namelist_test.pl fails to clean up

### Brief summary of bug

The cleanup at the end of `build-namelist_test.pl` fails with the following message:
```
Successfully ran all testing for build-namelist

Cleanup files created
/bin/rm: cannot remove 'temp_file.txt': No such file or directory
```

This can probably be avoided by adding the `-f` flag to all calls of `rm` during cleanup.

### General bug information

**CTSM version you are using:** ctsm5.3.010

**Does this bug cause significantly incorrect results in the model's science?** No

**Configurations affected:** n/a