net: don't append a dot to TXT records on Plan 9

TXT records are not domain names, so no need to call absDomainName.
The output now matches the pure Go resolver.

Fixes #44619

Change-Id: I1ebf09152ff5c0446d2e2b4c26671358892d9dc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/296589
Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: David du Colombier <0intro@gmail.com>
Trust: Ian Lance Taylor <iant@golang.org>

Author: fhs

src/net/lookup_plan9.go

@@ -308,7 +308,7 @@ func (*Resolver) lookupTXT(ctx context.Context, name string) (txt []string, err
 	}
 	for _, line := range lines {
 		if i := bytealg.IndexByteString(line, '\t'); i >= 0 {
-			txt = append(txt, absDomainName([]byte(line[i+1:])))
+			txt = append(txt, line[i+1:])
 		}
 	}
 	return