[REFACTOR] Remove unused parameters in Activity conversion methods

Since the (de)serialization (or the preperation thereof) of User and
SPath objects is no longer of the Activities' business, the
ISarosSession and IPathFactory parameters from the conversion methods
can be removed. 

Change-Id: Ia201de403df6bedb39970f883d8a0914417644c6
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1524
Tested-by: Jenkins CI
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>