Conservative elimination of !null checks before 'delete'

C++ guarantees that delete 0 and delete [] 0 do nothing.
Older buggy compilers that didn't honour this are no longer
supported by VTK.  Conservatively removed many "if (!ptr)"
checks before using delete in /Common.

Change-Id: Idf185d675e516e7fb5a8a745fd3dbf9720035325
Fix set-but-not-used warnings in Domains/Chemistry tests.

Change-Id: I54643fdc5079de1f2f52b51a94af5bfe9301f6ab
Merge topic 'chemistry-warnings' into master

41ddc334 Fix set-but-not-used warnings in Domains/Chemistry tests.
Merge topic 'if-before-delete' into master

890a9760 Conservative elimination of !null checks before 'delete'