borgbackup-static: only print warning if it is actually used

clanModules/borgbackup-static/default.nix

@@ -3,8 +3,6 @@ let
   clanDir = config.clan.core.clanDir;
   machineDir = clanDir + "/machines/";
 in
-lib.warn
-  "The borgbackup-static module is deprecated use the service via the inventory interface instead."
 {
   imports = [ ../borgbackup ];
 
@@ -100,4 +98,7 @@ lib.warn
       '';
     }
   ];
+  config.warnings = lib.optional (
+    builtins.length config.clan.borgbackup-static.targets > 0
+  ) "The borgbackup-static module is deprecated use the service via the inventory interface instead.";
 }