PluginBase: remove useless function

it's possible to check the loader instance instead of doing this, and we're gunning to get rid of the path from the plugin base.

src/pocketmine/plugin/PluginBase.php

@@ -254,13 +254,6 @@ abstract class PluginBase implements Plugin, CommandExecutor{
 		return false;
 	}
 
-	/**
-	 * @return bool
-	 */
-	protected function isPhar() : bool{
-		return strpos($this->file, "phar://") === 0;
-	}
-
 	/**
 	 * Gets an embedded resource on the plugin file.
 	 * WARNING: You must close the resource given using fclose()