CVE-2016-0771: dnsserver: don't force UTF-8 for TXT

While using a charset is not entirely logical, it allows testing of non
UTF-8 data (like inserting 0xFF into the TXT string).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11128
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11686

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
