remove a seemingly broken submodule

Committed unintentionally?
If not, please update .gitmodules next time.

diff --git a/deps/lightning/gnulib b/deps/lightning/gnulib
deleted file mode 160000 (submodule)
index e54b645..0000000
--- a/deps/lightning/gnulib
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit e54b645fc6b8422562327443bda575c65d931fbd