Remove lwip submodule because it is unused.

Author: tannewt

.gitmodules

@@ -5,9 +5,6 @@
 [submodule "lib/libffi"]
 	path = lib/libffi
 	url = https://github.com/atgreen/libffi
-[submodule "lib/lwip"]
-	path = lib/lwip
-	url = https://git.savannah.gnu.org/r/lwip.git
 [submodule "lib/berkeley-db-1.xx"]
 	path = lib/berkeley-db-1.xx
 	url = https://github.com/pfalcon/berkeley-db-1.xx