Get rid of patched version of Go 1.8.1

hack/lib/golang.sh

@@ -267,11 +267,6 @@ kube::golang::set_platform_envs() {
 
   # Do not set CC when building natively on a platform, only if cross-compiling from linux/amd64
   if [[ $(kube::golang::host_platform) == "linux/amd64" ]]; then
-    # We are currently using go 1.8.1, which has significant performance
-    # regression. Until this is fixed in Golang head, we are using patched
-    # version of Go that eliminates this problem.
-    export GOROOT=${K8S_PATCHED_GOROOT:-${GOROOT}}
-
     # Dynamic CGO linking for other server architectures than linux/amd64 goes here
     # If you want to include support for more server platforms than these, add arch-specific gcc names here
     case "${platform}" in