We disabled the original "colorized HTML edit-history" feature way
back in 2013 in c51056ff8e.
That original feature involved showing what had been edited inline in
message bodies, so one could easily see what had been changed.
That old feature has since been replaced with the "view edit history"
menu option, and we're unlikely to ever want the old feature back.
So, we can just remove its code. There's a few supporting variables
that were created to help implement this; we can clean those up and
simplify the `update_message` code now that this feature is fully
removed.
51018e12f5