simplify NET_LEGACY || NET_LWIP condition with NET condition

simplify NET_LEGACY || NET_LWIP condition with NET condition

Since the move to make NET a menuconfig and NO_NET a synonym of NET=n,
when NET is enabled, NET_LEGACY || NET_LWIP is necessarily true, so
let's simplify the various checks across the codebase.

SPL_NET_LWIP doesn't exist but SPL_NET_LEGACY is an alias for SPL_NET so
the proper symbol is still defined in SPL whenever needed.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>