Remove manifest from manifest.

It wasn't adding anything obviously useful, and .repo/manifests already
contains it, so we confusingly had two copies.

Change-Id: Ia2e308f743c5845d71ce349dfc3a7af87f2a571b
diff --git a/default.xml b/default.xml
index 6e0f2e2..85ef8e9 100644
--- a/default.xml
+++ b/default.xml
@@ -6,7 +6,6 @@
     review="https://gfiber-internal-review.googlesource.com/"/>
   <default remote="gfiber-internal" revision="master" sync-j="12"/>
 
-  <project name="manifests" path="manifest"/>
   <project name="toolchains/bruno"/>
   <project name="kernel/bruno"/>
   <project name="vendor/opensource/toolbox" path="toolbox-for-linux"/>