Allow verify-generated-docs to use go modcache

I don't think we need to use an entirely separate GOPATH, thus forcing
us to re-download modules every time we run `make`

This was causing verify-generated-docs to fail locally since go will
set its modcache as readonly and thus cause the cleanup "rm -rf" in
this script to fail. In go1.14 or later we could use "-modcacherw"
to stop making the modcache readonly but that bring me back to.. why
do need an entirely separate GOPATH in the first place?

hack/verify-generated-docs.sh

@@ -21,7 +21,6 @@ set -o pipefail
 CRT_DIR=$(pwd)
 VERIFY_TEMP=$(mktemp -d 2>/dev/null || mktemp -d -t k8s-community.XXXXXX)
 WORKING_DIR="${VERIFY_TEMP}/src/testgendocs"
-GOPATH="${VERIFY_TEMP}"
 mkdir -p "${WORKING_DIR}"
 
 function cleanup {