vulncheck: delete

This package has been moved to golang.org/x/vuln/vulncheck. It is
deleted here to prevent confusion.

Change-Id: I613fa3688489b1a8f1d8b933614edcc747deb34b
Reviewed-on: https://go-review.googlesource.com/c/exp/+/395256
Reviewed-by: Jonathan Amsterdam <[email protected]>
Trust: Julie Qiu <[email protected]>
Run-TryBot: Julie Qiu <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: julieqiu