net: remove the dreg of solaris merge

Also enables netgo test on solaris.

LGTM=aram, jsing
R=golang-codereviews, dave, aram, jsing
CC=golang-codereviews
https://golang.org/cl/70300043

src/pkg/net/cgo_unix.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build !netgo
-// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+// +build darwin dragonfly freebsd linux netbsd openbsd
 
 package net
 

src/pkg/net/netgo_unix_test.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build !cgo netgo
-// +build darwin dragonfly freebsd linux netbsd openbsd
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package net