simple pipeline: use default branch, not master

...especially since I recently switched from `master` to `main` on
sslip.io's repo.

Also I got rid of the Concourse groups, which I don't like at all. And I
added some pretty icons to the resources.

ci/pipeline-simple.yml

@@ -1,12 +1,6 @@
 # fly -t nono sp -p simple -c pipeline-simple.yml
 # fly -t nono expose-pipeline -p simple
 # fly -t nono unpause-pipeline -p simple
-groups:
-- name: simple
-  jobs:
-  - unit
-  - integration
-
 jobs:
 - name: unit
   plan:
@@ -40,6 +34,6 @@ jobs:
 resources:
 - name: src
   type: git
+  icon: github
   source:
     uri: https://github.com/cunnie/sslip.io
-    branch: master