doc: avoid references to removed config in sfc guide
doc: avoid references to removed config in sfc guide

CONFIG_* variables were used by make-based build system which is
removed.

Fixes: 3cc6ecfdfe85 ("build: remove makefiles")

Signed-off-by: Andrew Rybchenko <ac94ab2a8fe9a9a087f6e24dcbc16626b52c07e8@solarflare.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk