all: remove +build tags (#377)

Commit 759be3dad9 bumped our Go
requirement to 1.18 which allows us to drop support for old-style
`+build` tags. This change runs `go fix ./...` to remove them, and
updates some remaining code generators that produced `+build` lines.

examples/add/README.md

@@ -7,7 +7,6 @@ The [code generator](asm.go) is as follows:
 [embedmd]:# (asm.go)
 ```go
 //go:build ignore
-// +build ignore
 
 package main
 

examples/add/asm.go

@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main