Remove broken katacoda links from Charts example intros

[nicolethoen]

Many charts examples have a blurb like this at the top:
"Learn to build a bar chart using a Katacoda tutorial starting with a simple chart, adding multiple datasets, tooltips, axis labels, a legend, and concluding by changing the theme color. You'll learn how to use React chart components together to build a consistent user experience.

Start course"

this can all be removed, because katacoda does not exist anymore (until we have another plan for charts trainings).