Delete (broken) cmake submodule that somehow got in from my cmake branch

Signed-off-by: Cloudwalk <cloudwalk@icculus.org>

diff --git a/external/cmake b/external/cmake
deleted file mode 160000 (submodule)
index 7612abd..0000000
--- a/external/cmake
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 7612abd52f192a13848a4d74191633a008892449