This reverts commit 1956590df84a72c7f9a516e805d01529291fccf8.
Turns out the actual issue wasn't due to *invalid* links - it's with links
which have an unknown scheme.
There's still a change in behavior between Qt 5.10 and 5.11 though: Invalid
links are apparently not passed to acceptNavigationRequest (sometimes?) so we
don't show an error message. Instead, we just load about:blank. However,
Chromium does that too and we can't handle a real click easily, so let's just
ignore that one.
See #3661
0e9159e8e8