Dropped `prevent_deletion` handler.

An event handler for preventing object deletion is the wrong way, furhtermore
it makes impossible to remove repositories for administrators.

(cherry picked from commit 4a4bb1cc7400ac63a650c20bf372989def224a53)

Conflicts:
	docs/HISTORY.txt
Fixed mail upgradestep `to3401`, when it's run multipletimes.

With the current implementation, the new behavior gets doubled in the fti behaviors list
and makes some troubles when transporting to another clients (see `opengever.ogds.base.transporter`).

(cherry picked from commit e413ba0b7cd090df12db0e0fd703e4a3c5f84a2f)
Fixed mail upgradestep `to3402`, when it's run multipletimes.

With the current implementation, the classification gets doubled in the fti behaviors list
and makes some troubles when transporting to another clients (see `opengever.ogds.base.transporter`).

(cherry picked from commit a80130fb6868955e2755f83c563f972b866ce0c9)
Merge pull request #680 from 4teamwork/679_backport

Additional backports for Release 3.4.2