Remove primer/deploy action

[shawnbot]

This removes the primer/deploy action in favor of using Now's GitHub integration, which has some distinct advantages:

1. It uses the GitHub Deployments API, so the deployment links aren't buried in commit statuses.

2. It handles canceling in-progress builds if you push before they're done:

   With each new push, if Now is already building a previous commit, Now will cancel that current build to start the most recent commit so that you always have the latest changes deployed as quickly as possible.

3. It apparently creates automatic staging aliases for each pull request. I'm curious to see if this PR gets one, because Select Menu component #808 didn't. 🤔

Let's see how this goes!

[vercel]

This pull request is automatically deployed with Now.
To access deployments, click Details below or on the icon next to each push.

[shawnbot]

Re: the staging alias, the docs suggest that staging deployments will be automatically aliased using the following pattern:

But this URL doesn't seem to work:

primer-css-git-now-integration.primer.now.sh

➡️ Spectrum thread re: alias URLs

[shawnbot]

Confirmed:

The target aliasing is a Now 2.0 feature, unfortunately.
Are you sure you can't bump your repository to Now 2.0? Do you need any assistance?

If only it were that simple! 😭