No need to pass GOFLAGS to $goflags variables.

In go 1.11, go commands will use `GOFLAGS` as default flags, see
https://golang.org/doc/go1.11#go_command.

There is no need to pass GOFLAGS to $goflags, and if we do, go commands
will fail with "duplicate flags" error, e.g.

```
$ make test-integration WHAT=./test/integration/scheduler GOFLAGS="-v"
...
go test: v flag may be set only once
run "go help test" or "go help testflag" for more information
...
```

hack/lib/golang.sh

@@ -677,9 +677,7 @@ kube::golang::build_binaries() {
     local host_platform
     host_platform=$(kube::golang::host_platform)
 
-    # Use eval to preserve embedded quoted strings.
     local goflags goldflags goasmflags gogcflags
-    eval "goflags=(${GOFLAGS:-})"
     goldflags="${GOLDFLAGS:-} -s -w $(kube::version::ldflags)"
     goasmflags="-trimpath=${KUBE_ROOT}"
     gogcflags="${GOGCFLAGS:-} -trimpath=${KUBE_ROOT}"