greenkeeper: remove website from greenkeeper config

Description

website doesn't need vulnerability updates because it has no forms or real attack surface and it is not a published library that others depend on
- greenkeeper wasn't just for vulnerability updates though; automatic dep updates aren't necessary either as no one depends on it and it's not frequently used (until recently was last updated ~6 months ago)
greenkeeper is no longer a thing, but not sure if Snyk is perhaps using the same config, so try changing it to reduce all these unnecessary, duplicative, and buggy Snyk PRs
- based on the website / docs, it doesn't sound like this will change anything and this file should probably be deleted, but since I don't have access to Snyk to be able to change settings, this is all I can try