GeckoView does this link clearing stuff (which is somewhat dubious), but always
used to return NS_OK.
The error handling case in Link.cpp was pretty broken anyway (it'd leave the
link marked as registered), so make that infallible, given we fatally assert in
the non-android case.
NotifyVisited is also infallible so make that infallible too.
There's no need to use NS_IMETHOD gunk when not using scriptable XPCOM, and
these functions are not called from script, so make them regular pure-virtual
functions.
Depends on D50478.
Differential Revision: https://phabricator.services.mozilla.com/D50480
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extra : moz-landing-system : lando
f8863b0c94