General cleanup of DiracKernel stuff.

* Remove unused DiracKernelData object.
* Remove unnecessary includes from DiracKernel.h.

Refs #2847.
Merge pull request #2850 from jwpeterson/dirac

General cleanup of DiracKernel stuff.
Merge commit '56e8de90f80f242e7739780df5315d5a98e93e4b'