Remove the html version of the release note files for v600.

bindings/doc/v600/index.html

core/doc/v600/index.html

geom/doc/v600/index.html

graf2d/doc/v600/index.html

graf3d/doc/v600/index.html

gui/doc/v600/index.html

hist/doc/v600/index.html

hist/doc/v600/index.md

html/doc/v600/index.html

io/doc/v600/index.html

math/doc/v600/index.html

misc/doc/v600/index.html

montecarlo/doc/v600/index.html

net/doc/v600/index.html

proof/doc/v600/index.html

roofit/doc/v600/index.html

sql/doc/v600/index.html

tmva/doc/v600/index.html

tree/doc/v600/index.html

tutorials/doc/v600/index.html