remove unused theme getter from interface

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

src/main/java/com/nextcloud/client/preferences/AppPreferences.java

@@ -302,6 +302,4 @@ public interface AppPreferences {
     boolean isPowerCheckDisabled();
 
     void setPowerCheckDisabled(boolean value);
-
-    boolean getTheme();
 }