Move #isNpc method to PlayerUtils

- After dropping our hook to CombatTagPlus it is not relevant for it to be in PluginHooksService anymore

src/test/java/fr/xephi/authme/service/PluginHookServiceTest.java

@@ -58,8 +58,6 @@ public class PluginHookServiceTest {
         assertThat(pluginHookService.isEssentialsAvailable(), equalTo(true));
     }
 
-    // Note ljacqu 20160312: Cannot test with CombatTagPlus because its class is declared final
-
     @Test
     public void shouldHookIntoEssentialsAtInitialization() {
         // given