cmd/govulncheck: delete

This command now lives at golang.org/x/vuln/cmd/vulncheck.

Change-Id: I9c9dfca4660ad6088db2b66db5acc17e7bf12b9f
Reviewed-on: https://go-review.googlesource.com/c/exp/+/395257
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