CONTRIBUTING: Remove discussion of subtree workflow

We've dropped the subtree workflow with the introduction of the Gerrit
Owners plugin. Don't confuse people when instructing them how to
contribute.

Change-Id: Ia43bf13e9b70d0d90f5b08a9670ad7bce7a784d8
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 6136b4f..738ed81 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -273,9 +273,6 @@
 
   https://gerrit.openbmc-project.xyz/#/admin/projects/
 
-Please note that all meta-\* layers are separate repositories, so you need to
-follow the [subtree workflow](./subtree.md) when contributing there.
-
 If you're going to be working with Gerrit often, it's useful to create an SSH
 host block in ~/.ssh/config. Ex:
 ```