[cleanup] Remove some deprecated headers and classes.

Get rid of backwards-compatible AMGBackends, the P0fem
stuff and the outdated PDELab local operator wrapper.
(reviewed by fetzer)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14557 2fb0f335-1f38-0410-981e-8018bf24f1b0