stmmac: remove useless csum flag

This patch removes the no_csum_insertion private parameter that is not used anymore
and, also,  the "likely" annotation from the condition that is not in a critical path.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>