zephyr/prj_base.conf: Remove outdated CONFIG_NET_NBUF_RX_COUNT option.

CONFIG_NET_NBUF_RX_COUNT no longer exists in Zephyr, for a while. That
means we build with the default RX buf count for a while too, and it works,
so just remove it (instead of switching to what it was renamed to,
CONFIG_NET_PKT_RX_COUNT).

ports/zephyr/prj_base.conf

@@ -26,7 +26,6 @@ CONFIG_NET_UDP=y
 CONFIG_NET_TCP=y
 CONFIG_NET_SOCKETS=y
 CONFIG_TEST_RANDOM_GENERATOR=y
-CONFIG_NET_NBUF_RX_COUNT=5
 
 CONFIG_NET_APP_SETTINGS=y
 CONFIG_NET_APP_INIT_TIMEOUT=3