Removed OS from client ID

app/src/main/java/eu/faircode/email/EmailService.java

@@ -877,11 +877,10 @@ public class EmailService implements AutoCloseable {
     }
 
     static Map<String, String> getId(Context context) {
+        // https://www.rfc-editor.org/rfc/rfc2971.html
         Map<String, String> id = new LinkedHashMap<>();
         id.put("name", context.getString(R.string.app_name));
         id.put("version", BuildConfig.VERSION_NAME);
-        id.put("os", "Android");
-        id.put("os-version", Build.VERSION.RELEASE);
         return id;
     }