udhcp: comment out unused domain compression code

function                                             old     new   delta
attach_option                                        411     406      -5
dname_enc                                            381     167    -214
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(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-219)           Total: -219 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>