not used static final

Signed-off-by: tobiasKaminsky <tobias@kaminsky.me>

app/src/main/java/com/nextcloud/client/preferences/AppPreferencesImpl.java

@@ -92,7 +92,6 @@ public final class AppPreferencesImpl implements AppPreferences {
     private static final String PREF__PIN_BRUTE_FORCE_COUNT = "pin_brute_force_count";
     private static final String PREF__UID_PID = "uid_pid";
 
-    private static final String PREF__CALENDAR_AUTOMATIC_BACKUP = "calendar_automatic_backup";
     private static final String PREF__CALENDAR_LAST_BACKUP = "calendar_last_backup";
 
     private static final String PREF__PDF_ZOOM_TIP_SHOWN = "pdf_zoom_tip_shown";