Make clear() method of TrackingVolume non-const

TrackingVolume::clear() const (const is important) is responsible for a few mutable.

There is one possible use case of clearing a const volume, but the relevant flag seems to be always true, so the path !flag seems to not be used.

https://acode-browser1.usatlas.bnl.gov/lxr/search?%21v=head&_filestring=&_string=ResolveActiveLayers