Launch Seahorse, find your "login" encryption key, then right click on "Passwords: Login" and choose "Change Password".
Enter the old password, then hit the "Okay" button, leaving the new and confirm boxes empty. You will be prompted about "Unsafe Storage". Confirm this, and your keyring will be automatically unlocked when you log into your machine.
Note that this really is "Unsafe" and should only be used if you encrypt your home drive, as otherwise losing your laptop will equate to opening up everything it has access to - that might include your Gmail password (if you use a Gmail checker) your WIFI passwords if you connect to WIFI, IM passwords and so on. Looks like Chromium has started using it for storage too, as I seem to have a lot of site-specific stuff recorded in my keyring.
Personally I also uninstall Seahorse after setting everything up too, so that it's a little less trivial to see all my passwords in cleartext should I forget to lock my laptop!
Be careful with unsafe storage.