Remove reference to expired push subscriptions

[beverloo]

Chrome does not expire push subscriptions, and this sentence is causing developer confusion. The CTA for this error case is unaffected.

What's changed, or what was fixed?

• Remove reference to expired subscriptions from push notification docs.

Fixes: N/A

Target Live Date: Whenever.

• This has been reviewed and approved by (NAME)
• I have run npm test locally and all tests pass.
• I have added the appropriate type-something label.
• I've staged the site and manually verified that my content displays correctly.

CC: @petele @PaulKinlan

[WebFundBot]

🎉 This has been pushed live to https://developers.google.com/web/