Remove all BTX and ETX markers from VTK headers

Perl was used to remove all the BTX and ETX markers from the code. The specific command that was run on all "vtk*.h" files was this:

perl -0777 -i -pe 's/(\n* *\/\/ *[BE]TX *\n+)+/\n\n/g'

This regex replaces each BTX/ETX line, plus any leading or trailing blank lines, with a single blank line.