Remove default binding for LinkHints.activateModeToDownloadLink.

LinkHints.activateModeToDownloadLink is newly-added, and hasn't been in
a release before.  It's an advanced command, and it's not clear it
warrants a default binding.
Merge pull request #1309 from smblott-github/no-default-binding-for-download-link

Remove default binding for LinkHints.activateModeToDownloadLink.