Remove unnecessary null pointer checks.

Explicit null pointer checks are performed at some source code places before pointers are passed to the C++ delete operator. This is unnecessary because [the delete operator will care for such parameter validation already](http://www.parashift.com/c++-faq/delete-handles-null.html "Do I need to check for NULL before delete p? ").

This is a follow-up to [a previous update suggestion](https://github.com/libMesh/libmesh/issues/169 "https://github.com/libMesh/libmesh/issues/169").