Fix remove unnecessary null pointer checks, issue #257.

There only appeared to be one obvious case left.

Code search process:

    grep -C 3 delete .

... then look for if statements, in C-like code, conditioned on the same
variable, and no other obvious purpose that required the if statement.

Signed-off-by: Eric Powers <[email protected]>

Author: epowers

nanovdb/nanovdb/tools/GridBuilder.h

@@ -1968,7 +1968,7 @@ void Grid<BuildT>::operator()(const Func& func, const CoordBBox& bbox, ValueType
                 NANOVDB_ASSERT(leaf == nullptr);
             }
         }// loop over sub-part of leafBBox
-        if (leaf) delete leaf;
+        delete leaf;
     });
 
     // Prune leaf and tile nodes