end the #commits-behind=0 experiment

The problem with it turns out to be that you get no warning at all
if it's waiting to be manually rebased; you have to inspect the
Mergify state to find out. At least with the old behavior you get
am error when Mergify tries to rebase it, and you know why.

.github/mergify.yml

@@ -97,10 +97,6 @@ pull_request_rules:
       - label=merge delay passed
       - '#approved-reviews-by>=2'
       - '-label~=^blocked:'
-      # unlike the others, we need to force this one to be up to date
-      # because it's intended for when Mergify doesn't have permission
-      # to rebase
-      - '#commits-behind=0'
 
   # merge strategy for release branches
   - actions: