Delete duplicate icon in notifications.html

[TimvdLippe]

Today I was working on this element and I discovered that the tests actually failed. This was because the icon name rv-hookup was declared twice: once in places and once in notifications. Looking at https://material.io/icons/#ic_rv_hookup, the places icon is correct, but the notification:rv-hookup is incorrect. As such, remove the rv-hookup in notification.html and make the test pass again.

[notwaldorf]

These files are automatically generated, using the script in this repo. If we merge this change, it will just get stomped on next time we run the script

[TimvdLippe]

Oh I did not know. Then it appears the issue is at https://github.com/google/material-design-icons/blob/master/notification/svg/production/ic_rv_hookup_24px.svg and it seems the issue is reported at https://github.com/google/material-design-icons/issues/415

Would be nice to add a note of the generation script in the README to prevent PRs like this one to be published 😄