Fix verify: generated-stable-metrics wrt go 1.18

This should fix the following error when running
./hack/update-generated-stable-metrics.sh:

	'go get' is no longer supported outside a module.
	To build and install a command, use 'go install' with a version,
	like 'go install example.com/cmd@latest'
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

Using `go get` to download gopkg.in/yaml.v2 package into
KUBE_EXTRA_GOPATH directory no longer works. Interestingly, main repo
already has gopkg.in/yaml.v2@v2.4.0, same version that was installed by
that go get.

I guess that GOPATH with multiple elements no longer works either,
and since this code was the only user of KUBE_EXTRA_GOPATH, let's remove
it as well.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

hack/lib/golang.sh

@@ -496,9 +496,6 @@ EOF
 # ${PATH}. It will also check that the Go version is good enough for the
 # Kubernetes build.
 #
-# Inputs:
-#   KUBE_EXTRA_GOPATH - If set, this is included in created GOPATH
-#
 # Outputs:
 #   env-var GOPATH points to our local output dir
 #   env-var GOBIN is unset (we want binaries in a predictable place)
@@ -512,11 +509,6 @@ kube::golang::setup_env() {
   export GOPATH="${KUBE_GOPATH}"
   export GOCACHE="${KUBE_GOPATH}/cache"
 
-  # Append KUBE_EXTRA_GOPATH to the GOPATH if it is defined.
-  if [[ -n ${KUBE_EXTRA_GOPATH:-} ]]; then
-    GOPATH="${GOPATH}:${KUBE_EXTRA_GOPATH}"
-  fi
-
   # Make sure our own Go binaries are in PATH.
   export PATH="${KUBE_GOPATH}/bin:${PATH}"