[StokesLocalResidual] Revert renaming of variable from r9781.

There I introduces a bug by naming a loop variable that was already
declared. Kudos to fetzer for doing all the work and finding this bug.
The Stokes models should convert better again.
(reviewed by bernd)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10041 2fb0f335-1f38-0410-981e-8018bf24f1b0