net: dns: Remove unused variable

Fix compilation warnings:
...
subsys/net/lib/dns/dns_pack.c:548:6: warning: variable
'remaining_size' set but not used [-Wunused-but-set-variable]
        int remaining_size;
            ^
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>