build: remove build constraint syntax for go 1.16 and older

cmd/selfupdate/noselfupdate.go

@@ -1,5 +1,4 @@
 //go:build noselfupdate
-// +build noselfupdate
 
 package selfupdate
 

cmd/selfupdate/selfupdate.go

@@ -1,5 +1,4 @@
 //go:build !noselfupdate
-// +build !noselfupdate
 
 // Package selfupdate provides the selfupdate command.
 package selfupdate

cmd/selfupdate/selfupdate_test.go

@@ -1,5 +1,4 @@
 //go:build !noselfupdate
-// +build !noselfupdate
 
 package selfupdate
 

cmd/selfupdate/verify.go

@@ -1,5 +1,4 @@
 //go:build !noselfupdate
-// +build !noselfupdate
 
 package selfupdate
 

cmd/selfupdate/verify_test.go

@@ -1,5 +1,4 @@
 //go:build !noselfupdate
-// +build !noselfupdate
 
 package selfupdate
 

cmd/selfupdate/writable_unix.go

@@ -1,5 +1,4 @@
 //go:build !windows && !plan9 && !js && !noselfupdate
-// +build !windows,!plan9,!js,!noselfupdate
 
 package selfupdate
 

cmd/selfupdate/writable_unsupported.go

@@ -1,6 +1,4 @@
 //go:build (plan9 || js) && !noselfupdate
-// +build plan9 js
-// +build !noselfupdate
 
 package selfupdate
 

cmd/selfupdate/writable_windows.go

@@ -1,5 +1,4 @@
 //go:build windows && !noselfupdate
-// +build windows,!noselfupdate
 
 package selfupdate