Attribute longdesc shouldnt check

[taicomjp]

I found linkchecker check attribute longdesc, I think it shouldnt be check, its not url

[dpalic]

this project is dead, please see: #708

[dpalic]

Thank you for the issue report. Sadly this project is dead, and a new team is around with https://github.com/linkcheck/linkchecker
for more details please see: #708
Also please close this issue and report it freshly on the new repo https://github.com/linkcheck/linkchecker/issues