WebGPURenderer: Fix duplicate shader programs in the WebGLBackend (#29135)

Co-authored-by: aardgoose <angus.sawyer@email.com>

src/renderers/webgl-fallback/WebGLBackend.js

@@ -747,9 +747,9 @@ class WebGLBackend extends Backend {
 
 	}
 
-	getRenderCacheKey( renderObject ) {
+	getRenderCacheKey( /*renderObject*/ ) {
 
-		return renderObject.id;
+		return '';
 
 	}