วิธีแก้ GPG error: http://dl.google.com/linux/mod-pagespeed/deb stable

ubuntu ลบ mod_pagespeed ออกแล้ว เมื่อ apt update แล้วพบ error

W: GPG error: http://dl.google.com/linux/mod-pagespeed/deb stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6494C6D6997C215E
E: The repository 'http://dl.google.com/linux/mod-pagespeed/deb stable Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

ให้ทำการลบไฟล์ mod-pagespeed.list ในโฟลเดอร์ /etc/apt/sources.list.d

rm /etc/apt/sources.list.d/mod-pagespeed.list

@iPepzi.