Don't allow ensure=file anymore

@cocker-cc correctly mentioned in
7f63dfa#commitcomment-139614892
that after #405 `ensure=file` breaks due to `stdlib::ensure` not being
able to handle that. As `file` is any way only a synonym for `present`,
let's just remove it from the possible values.

manifests/unit_file.pp

@@ -66,7 +66,7 @@
 #   }
 #
 define systemd::unit_file (
-  Enum['present', 'absent', 'file']        $ensure    = 'present',
+  Enum['present', 'absent']                $ensure    = 'present',
   Stdlib::Absolutepath                     $path      = '/etc/systemd/system',
   Optional[Variant[String, Sensitive[String], Deferred]] $content = undef,
   Optional[String]                         $source    = undef,