Disabled ws cleanup for obs-event-dispatcher job

Disabled ws cleanup for obs-event-dispatcher job

Workspace cleanup plugin doesn't work properly when job allows parallel
build execution. It removes workspaces for other builds, which causes
spectacular crashes like this:
[workspace@2] $ /bin/bash /tmp/hudson934458048082312142.sh
python: can't open file
'/var/lib/jenkins/jobs/obs-event-dispatcher/workspace@2/jenkins-scripts/job_jobs_dispatcher.py':
[Errno 2] No such file or directory
Build step 'Execute shell' marked build as failure

This change replaces buggy magic of workspace cleanup plugin with simple
deletion of *.env files.

Fixes: #1328

Change-Id: I7f9b59dcc10d11ba985f6ae58933d6d250e4b820
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>