net: stmmac: Drop useless code related to ethtool rx-copybreak

net: stmmac: Drop useless code related to ethtool rx-copybreak

After commit 2af6106ae949 ("net: stmmac: Introducing support for Page
Pool"), the driver always copies frames to get a better performance,
zero-copy for RX frames is no more, then these code turned to be
useless and users of ethtool may get confused about the unhandled
rx-copybreak parameter.

This patch mostly reverts
commit 22ad38381547 ("stmmac: do not perform zero-copy for rx frames")

Signed-off-by: Furong Xu <0x1207@gmail.com>
Link: https://patch.msgid.link/20241218083407.390509-1-0x1207@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>