Remove default binding for LinkHints.activateModeToDownloadLink.
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.

justalittlenoob/vimium,douglas-larocca/vimium,mrmr1993/vimium,gdh1995/vimium,cyrixhero/vimium,dimatter/vimium,PickRelated/vimium,Aleroniponi/vimium,hhsue/vimium,philc/vimium,dimatter/vimium,zhangmin510/vimium,douglas-larocca/vimium,willsALMANJ/vimium,mrmr1993/vimium,PrestanceDesign/vimium,bkudria/vimium,elderbas/vimium,bkudria/vimium,crazypenguincode/vimium,PrestanceDesign/vimium,smblott-github/vimium,bkudria/vimium,elderbas/vimium,tuchangwei/vimium,borgified/vimium,decaffeinate-examples/vimium,elegantwww/vimium,borgified/vimium,wildeyes/vimium,Robinson10240/vimium,csmalin/vimium,wildeyes/vimium,Aleroniponi/vimium,PickRelated/vimium,cyrixhero/vimium,cyrixhero/vimium,gdh1995/vimium,smblott-github/vimium,elegantwww/vimium,poacher2k/vimium,Aleroniponi/vimium,dimatter/vimium,borgified/vimium,elegantwww/vimium,csmalin/vimium,Lw-Cui/vimium,VPashkov/vimium,PickRelated/vimium,poacher2k/vimium,douglas-larocca/vimium,willsALMANJ/vimium,willsALMANJ/vimium,hhsue/vimium,wildeyes/vimium,tuchangwei/vimium,csmalin/vimium,decaffeinate-examples/vimium,VPashkov/vimium,tuchangwei/vimium,philc/vimium,mrmr1993/vimium,crazypenguincode/vimium,zhangmin510/vimium,hhsue/vimium,zhangmin510/vimium,elderbas/vimium,philc/vimium,justalittlenoob/vimium,VPashkov/vimium,justalittlenoob/vimium,Robinson10240/vimium,Lw-Cui/vimium,Lw-Cui/vimium,smblott-github/vimium,Robinson10240/vimium,decaffeinate-examples/vimium,PrestanceDesign/vimium,gdh1995/vimium,poacher2k/vimium