vulndb: delete

Delete the vulndb module. Users should now use
golang.org/x/vuln/cmd/govulncheck.

Change-Id: Ia592a36f410fd7aef35a45b077cb2cbbf4bec38c
Reviewed-on: https://go-review.googlesource.com/c/exp/+/395534
Reviewed-by: Zvonimir Pavlinovic <[email protected]>
Reviewed-by: Jonathan Amsterdam <[email protected]>
Trust: Julie Qiu <[email protected]>

Author: julieqiu