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>

Author: tklauser

plan9/pwd_go15_plan9.go

@@ -2,22 +2,18 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !go1.5
-
 package plan9
 
+import "syscall"
+
 func fixwd() {
+	syscall.Fixwd()
 }
 
 func Getwd() (wd string, err error) {
-	fd, err := open(".", O_RDONLY)
-	if err != nil {
-		return "", err
-	}
-	defer Close(fd)
-	return Fd2path(fd)
+	return syscall.Getwd()
 }
 
 func Chdir(path string) error {
-	return chdir(path)
+	return syscall.Chdir(path)
 }