Remove the full releng submodule

Just removing releng from .gitmodules is not enough.

Change-Id: Ic275d6b87ef60fe24b5be8c0142f4a704b666b2b
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>

diff --git a/docs/submodules/releng b/docs/submodules/releng
deleted file mode 160000 (submodule)
index 654bd03..0000000
--- a/docs/submodules/releng
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 654bd03af21b420227068465dc7cb89ab471b308