contrib/kube-prometheus: add `-f` to `rm` in `build.sh`

According to the man pages of `rm` and the `-f` option:
> Attempt to remove the files without prompting for confirma- tion,
> regardless of the file's permissions.  If the file does not exist, do
> not display a diagnostic message or modify the exit status to reflect an
> error.  The -f option overrides any previous -i options.

This patch prevents the `build.sh` script from failing if files do not
exist when attempting to delete them.