net: fec: remove the remaining code of rx copybreak

Since the commit 95698ff6 ("net: fec: using page pool to manage
RX buffers") has been applied, all the rx packets, no matter small
packets or large packets are put directly into the kernel networking
buffers. That is to say, the rx copybreak function has been removed
since then, but the related code has not been completely cleaned up.
So the purpose of this patch is to clean up the remaining related
code of rx copybreak.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://lore.kernel.org/r/20230718090928.2654347-2-wei.fang@nxp.com


Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>