fix(ai): drop explanatory comment in getMediaTypeFromUrl

Per review on vercel#14751: the regression test for the prototype-property
collision case is enough; the inline comment is redundant.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: etairl

packages/ai/src/prompt/convert-to-language-model-prompt.ts

@@ -742,9 +742,6 @@ function getMediaTypeFromUrl(
   try {
     const pathname = new URL(url).pathname;
     const ext = pathname.split('.').pop()?.toLowerCase();
-    // Use `Object.hasOwn` instead of `in` so attacker-controlled extensions
-    // like `constructor` cannot resolve to inherited `Object.prototype`
-    // members and leak a non-string value through this `: string` helper.
     if (ext && Object.hasOwn(URL_EXTENSION_TO_MEDIA_TYPE, ext)) {
       return URL_EXTENSION_TO_MEDIA_TYPE[ext];
     }