MAINTAINERS: stmmac: retire Giuseppe Cavallaro

I tried to get stmmac maintainers to be more active by agreeing with
them off-list on a review rotation. I pinged Peppe 3 times over 2 weeks
during his "shift month", no reviews are flowing.

All the contributions are much appreciated! But stmmac is quite
active, we need participating maintainers :(

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230726151120.1649474-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>