Remove 'devel' branch usage recommendation

We are consolidating development into the 'main' branch instead of
having the 'devel' intermediary. As such, remove the advice to submit
pull requests to the 'devel' branch.

Author: d-e-s-o

doc/CONTRIBUTING.md

@@ -1,11 +1,5 @@
 The following rules generally apply for pull requests and code changes:
 
-**Submit Pull Requests to the `devel` branch**
-
-The `devel` branch is where experimental features reside. After some
-soak time they may be ported over to `main` and a release will be cut
-that includes them.
-
 **Keep documentation up-to-date**
 
 Please make an effort to keep the documentation up-to-date to the extent