net: amt: Move stats allocation to core

With commit 34d21de9

 ("net: Move {l,t,d}stats allocation to core and
convert veth & vrf"), stats allocation could be done on net core instead
of this driver.

With this new approach, the driver doesn't have to bother with error
handling (allocation failure checking, making sure free happens in the
right spot, etc). This is core responsibility now.

Move amt driver to leverage the core allocation.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20240308162606.1597287-1-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>