jobs: gerrit: fix: don't do git plugin cleanup

After the "manual" cleanup from I2409b2928b4d7ebbd6c005097d4ad7337307dd93 has
been applied in numerous jenkins.sh scripts of the various osmocom projects,
drop the git plugin cleanup step.

Rationale:
- The git plugin cleanup apparently does not check against non-writable files,
  like leftovers from a failed 'make distcheck'. If any are left behind, the
  build fails even before the build steps start.
- The git cleanup would wipe out all git clones, which we're trying to re-use
  across rebuilds now.

The osmo-clean-workspace.sh takes care of this, so don't use the jenkins
plugin.

Change-Id: I8a43b851cef2ad7e8582f1e9bbe3547e3b77fc1a

jobs/gerrit-verifications.yml

@@ -229,8 +229,6 @@
          choosing-strategy: gerrit
          wipe-workspace: false
          skip-tag: true
-         clean:
-           before: true
 
     triggers:
       - gerrit: