build: remove build constraint syntax for go 1.16 and older

cmd/ncdu/ncdu.go

@@ -1,5 +1,4 @@
 //go:build !plan9 && !js
-// +build !plan9,!js
 
 // Package ncdu implements a text based user interface for exploring a remote
 package ncdu

cmd/ncdu/ncdu_unsupported.go

@@ -2,6 +2,5 @@
 // about "no buildable Go source files "
 
 //go:build plan9 || js
-// +build plan9 js
 
 package ncdu