internal/cmd/govulncheck: remove unnecessary binary dependency

This is simply not needed anymore.

Change-Id: I02cf1459f060ce8522655107c641e7c3f972e231
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/590015
Run-TryBot: Zvonimir Pavlinovic <zpavlinovic@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Maceo Thompson <maceothompson@google.com>

cmd/govulncheck/integration/Dockerfile

@@ -12,6 +12,5 @@ WORKDIR /go/src/golang.org/x/vuln/cmd/govulncheck/integration
 RUN go install golang.org/x/vuln/cmd/govulncheck
 
 # ---- Step 2: Build other test binaries ----
-RUN go install golang.org/dl/go1.18@latest
 RUN go install golang.org/x/vuln/cmd/govulncheck/integration/k8s
 RUN go install golang.org/x/vuln/cmd/govulncheck/integration/stackrox-scanner