net: dsa: tag_ar9331: let DSA core deal with TX reallocation

Now that we have a central TX reallocation procedure that accounts for
the tagger's needed headroom in a generic way, we can remove the
skb_cow_head call.

Cc: Per Forlin <per.forlin@axis.com>
Cc: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>