refactor: remove method, which can be replaced with get_path

Signed-off-by: Bart Jaskulski <bjaskulski@protonmail.com>

src/Plugin.php

@@ -78,13 +78,6 @@ final class Plugin {
 		return $this->basename;
 	}
 
-	/**
-	 * Retrieve the plugin directory.
-	 */
-	public function get_directory(): string {
-		return $this->directory;
-	}
-
 	/**
 	 * Retrieve the path to a file in the plugin.
 	 *