There still left some references to GtkHTML in the "active code",
which should be dropped. There still can be found couple "GtkHTML"
strings in the code, but mostly in disabled code or comments only.
These are to be fixed separately.
Turns out this function was a no-op, because nothing registered itself
to be cancelled this way. This was part of the legacy async framework,
which we've mostly moved away from now.
Milan Crha