[UI|INTERNAL] removed option to secure preference store

This option became obsolete with commit
2a48c3794325c017192babc3b9a3fa4db721adc4.

Also moved the conversion method back to the Saros class.

Change-Id: I9be3cb3473356e9f905c1d7c5d1a977985510582
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1635
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>