Remove setCustomJs of the files_external FrontendDefinitionTrait

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

apps/files_external/lib/Lib/FrontendDefinitionTrait.php

@@ -106,16 +106,6 @@ trait FrontendDefinitionTrait {
 		return $this;
 	}
 
-	/**
-	 * @param string $custom
-	 * @return self
-	 * @deprecated 9.1.0, use addCustomJs() instead
-	 */
-	public function setCustomJs($custom) {
-		$this->customJs = [$custom];
-		return $this;
-	}
-
 	/**
 	 * Serialize into JSON for client-side JS
 	 *