[cmake,release] Removes bogus link directory.

Somehow we ended up putting CMAKE_PROJECT_SOURCE_DIR/lib into the
linker path. There should not be any library there and it should
not be needed for linking anyway. Additionally it causes warnings
in dune-grid-howto. Therefore this patch simply removes it.
(cherry picked from commit dc0673bad1f666564f199c56602f80e921be5615)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>