plan9: drop go version tags for unsupported versions

go.mod specifies go 1.24. Drop code for older, unsupported versions.

Change-Id: I8a1ecd9e5634c1cf9619fbb89b2ecd0bda7eaf21
Reviewed-on: https://go-review.googlesource.com/c/sys/+/579515
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>