Remove unused property myBox of the class BRepClass3d_SolidExplorer

The class BRepClass3d_SolidExplorer has the field myBox, and it is computed during initialization of the class instance. But this property is never used in the current configuration.

It is used only if preprocessor definition MARCHEPASSIUNESEULEFACE is defined in the file BRepClass3d_SolidClassifier.cxx.

I propose to put the codes of BRepClass3d_SolidClassifier and BRepClass3d_SolidExplorer in correspondence with the only definition, and to put existence of the field myBox and the method Box() under the same condition.