Remove workaround for Nano server TP5

This workaround for golang/go#15286 was added for Nano server TP5 in
fa82c0aa10cfac8c6d5e2446876dc79b2b0c1bf9, and should no longer be
needed

Due to a security fix in Go 1.9.4/1.8.7, loading the .dll is no longer
allowed, and produces an error:

   .\docker_windows.go:9:3: //go:cgo_import_dynamic main.dummy CommandLineToArgvW%2 "shell32.dll" only allowed in cgo-generated code

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cmd/dockerd/docker_windows.go

@@ -1,18 +1,5 @@
 package main
 
 import (
-	"sync/atomic"
-
 	_ "github.com/docker/docker/autogen/winresources/dockerd"
 )
-
-//go:cgo_import_dynamic main.dummy CommandLineToArgvW%2 "shell32.dll"
-
-var dummy uintptr
-
-func init() {
-	// Ensure that this import is not removed by the linker. This is used to
-	// ensure that shell32.dll is loaded by the system loader, preventing
-	// go#15286 from triggering on Nano Server TP5.
-	atomic.LoadUintptr(&dummy)
-}