STYLE: Simplify metadata stripping

Rather than iterating over the whole metadata and copying keys one at a
time if not in the 'ignore' set, just copy the whole thing en-masse and
remove the unwanted values. This gives simpler code.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23170 3bd1e089-480b-0410-8dfb-8563597acbee