Remove source from pipelines (2/2)

Now that sources are associated with projects, remove the source
attribute from pipelines altogether.

This removes the occurance in tests fixtures as well as the element
from the pipeline configuration schema.

Change-Id: If47ecfcd5c5c82b30f6ee8890ed0766cfc88739e
Story: 2000953

tests/fixtures/config/in-repo/git/common-config/zuul.yaml

@@ -1,7 +1,6 @@
 - pipeline:
     name: check
     manager: independent
-    source: gerrit
     trigger:
       gerrit:
         - event: patchset-created
@@ -16,7 +15,6 @@
     name: tenant-one-gate
     manager: dependent
     success-message: Build succeeded (tenant-one-gate).
-    source: gerrit
     trigger:
       gerrit:
         - event: comment-added