drop gnulib submodule

The upstream server is just unreliable and it annoyingly breaks the CI.
I'd rather apply this on every lightning merge than have a broken CI.

Author: notaz

.gitmodules

@@ -1,7 +1,3 @@
 [submodule "frontend/libpicofe"]
 	path = frontend/libpicofe
 	url = https://github.com/notaz/libpicofe.git
-[submodule "deps/lightning/gnulib"]
-	active = false
-	path = deps/lightning/gnulib
-	url = https://git.savannah.gnu.org/git/gnulib.git