Remove workaround to perf issues in go1.6

Starting in go1.9, Golang lowers the timer frequency when not required
reducing CPU usage. This workaround prevents Golang from doing so,
and since the initial issue was fixed, I see no perf regressions.
This does not need to be conditional based on Golang verison, as the issue
being worked around was only in go1.6.

Signed-off-by: Darren Stahl <darst@microsoft.com>

file.go

@@ -16,7 +16,6 @@ import (
 //sys createIoCompletionPort(file syscall.Handle, port syscall.Handle, key uintptr, threadCount uint32) (newport syscall.Handle, err error) = CreateIoCompletionPort
 //sys getQueuedCompletionStatus(port syscall.Handle, bytes *uint32, key *uintptr, o **ioOperation, timeout uint32) (err error) = GetQueuedCompletionStatus
 //sys setFileCompletionNotificationModes(h syscall.Handle, flags uint8) (err error) = SetFileCompletionNotificationModes
-//sys timeBeginPeriod(period uint32) (n int32) = winmm.timeBeginPeriod
 
 type atomicBool int32
 
@@ -153,8 +152,6 @@ func (f *win32File) prepareIo() (*ioOperation, error) {
 
 // ioCompletionProcessor processes completed async IOs forever
 func ioCompletionProcessor(h syscall.Handle) {
-	// Set the timer resolution to 1. This fixes a performance regression in golang 1.6.
-	timeBeginPeriod(1)
 	for {
 		var bytes uint32
 		var key uintptr

zsyscall_windows.go

@@ -38,14 +38,12 @@ func errnoErr(e syscall.Errno) error {
 
 var (
 	modkernel32 = windows.NewLazySystemDLL("kernel32.dll")
-	modwinmm    = windows.NewLazySystemDLL("winmm.dll")
 	modadvapi32 = windows.NewLazySystemDLL("advapi32.dll")
 
 	procCancelIoEx                                           = modkernel32.NewProc("CancelIoEx")
 	procCreateIoCompletionPort                               = modkernel32.NewProc("CreateIoCompletionPort")
 	procGetQueuedCompletionStatus                            = modkernel32.NewProc("GetQueuedCompletionStatus")
 	procSetFileCompletionNotificationModes                   = modkernel32.NewProc("SetFileCompletionNotificationModes")
-	proctimeBeginPeriod                                      = modwinmm.NewProc("timeBeginPeriod")
 	procConnectNamedPipe                                     = modkernel32.NewProc("ConnectNamedPipe")
 	procCreateNamedPipeW                                     = modkernel32.NewProc("CreateNamedPipeW")
 	procCreateFileW                                          = modkernel32.NewProc("CreateFileW")
@@ -122,12 +120,6 @@ func setFileCompletionNotificationModes(h syscall.Handle, flags uint8) (err erro
 	return
 }
 
-func timeBeginPeriod(period uint32) (n int32) {
-	r0, _, _ := syscall.Syscall(proctimeBeginPeriod.Addr(), 1, uintptr(period), 0, 0)
-	n = int32(r0)
-	return
-}
-
 func connectNamedPipe(pipe syscall.Handle, o *syscall.Overlapped) (err error) {
 	r1, _, e1 := syscall.Syscall(procConnectNamedPipe.Addr(), 2, uintptr(pipe), uintptr(unsafe.Pointer(o)), 0)
 	if r1 == 0 {