Drop packets with garbage at the end on the floor

This effectively reverts 170736db76139ed9fff9dbf70a55d4ba4f25d9bd. That commit
didn't work anyway. It fails to update packet_len, so we computed the
Z_InputQ's header_len wrong and fail to truncate the garbage anyway. Plus
packets like that likely come from a broken cross-realm zephyrd without
f276622ace757977fec43633e43577350e0cf6fe, so we want to drop them.

That patch has yet to be in a released libzephyr, so if there are other sources
of notices with trailing garbage, no one was relying on them working anyway.
On the contrary, we were relying on them NOT working in that it masks broken
cross-realm zephyrds.