BUG: Remove check of spacing in LabelStatistics

Because small differences in spacing can be introduced by rounding
or the way floating point numbers are handled, the check of exact
equality caused false negatives.  Removing this check will allow
valid use cases to succeed, while the other checks still prevent
incorrect calculations (where the grids to not actually align in
pixel space).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22024 3bd1e089-480b-0410-8dfb-8563597acbee