net: ipv6: mld: remove addition of unspecified source in MLDv2 report

From RFC:
* "source list" is an unordered list of zero or more unicast
addresses from which multicast reception is desired or not desired [...]

It means that there is no need to add any special address to specify
"block all" and "allow all" states explicitly. This commit removes the
unnecessary addition.

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>