replace imported ContentClick module with stub

replace imported ContentClick module with stub

The module was replaced at the very end of the 72 cycle with
ClickHandlerParent.jsm, which made contentAreaClick non-static. It would
have been technically possible to just ship the old module with either
VimFx or LegacFox, but since it is so small and it was replaced to
support Fission, it seemed better to extract the relevant parts of the
method and just inline them. We'll need to keep it up-to-date either
way.
Note that despite this we are _not_ fission compatible; when a 3rd-party
iframe is present, hints mode breaks.

Regressed by: Bug 1595155