net: ethernet: Remove L2 header stripping after TX

It seems that this change was solely added to address issues with old
TCP stack, which blindly queued packets intended for TX for potential
further retransmission, expecting that the packet would remain intact
during transmission.

I think this assumption was wrong, as it's natural that lower layers
append respective headers to the packet, and this "header stripping"
behavior was specific for Ethernet L2 only. If an upper layer expects
that the packet would need to be retransmitted at some point, it should
clone it instead.

Therefore, remove the L2 header stripping from the Ethernet L2 to avoid
any potential issues in zero-copy case.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>