safesocket: delete incorrect build tag

Leftover from copying one file to another to create safesocket.go
in the earlier documentation change (29f7d64091).

safesocket/safesocket.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !windows
-
 // Package safesocket creates either a Unix socket, if possible, or
 // otherwise a localhost TCP connection.
 package safesocket