unix: drop go version tags for unsupported versions

go.mod specifies go 1.18. Drop go1.n version tags for older, unsupported
versions.

Change-Id: I8563bd2cc8e86bd560a45b885a9aa2e6bcc95c47
Reviewed-on: https://go-review.googlesource.com/c/sys/+/559415
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/darwin_amd64_test.go

@@ -1,7 +1,7 @@
 // go run mkasm.go darwin amd64
 // Code generated by the command above; DO NOT EDIT.
 
-//go:build darwin && go1.12
+//go:build darwin
 
 package unix