remove PluginBase->getFile()

this is in preparation for opening the doors to more exotic kinds of plugin loaders, such as eval()d or web plugins.

src/pocketmine/plugin/PluginBase.php

@@ -370,13 +370,6 @@ abstract class PluginBase implements Plugin, CommandExecutor{
 		return $this->description->getFullName();
 	}
 
-	/**
-	 * @return string
-	 */
-	protected function getFile() : string{
-		return $this->file;
-	}
-
 	/**
 	 * @return PluginLoader
 	 */