lwIP: Remove redundant lowspan6 source files

This is a preparation for adding 6LoWPAN config options. We would
optionally add:
*       "lwip/src/netif/lowpan6_common.c"
*       "lwip/src/netif/lowpan6.c"
*       "lwip/src/netif/lowpan6_ble.c"
But only if LWIP_LOWPAN6_SUPPORT is enabled in lwip menuconfig.

Merges https://github.com/espressif/esp-idf/pull/7436

components/lwip/CMakeLists.txt

@@ -55,7 +55,6 @@ set(srcs
     "lwip/src/core/ipv6/mld6.c"
     "lwip/src/core/ipv6/nd6.c"
     "lwip/src/netif/ethernet.c"
-    "lwip/src/netif/lowpan6.c"
     "lwip/src/netif/slipif.c"
     "lwip/src/netif/ppp/auth.c"
     "lwip/src/netif/ppp/ccp.c"