[Onion Services] Remove mentions of v2 onions in 'I cannot reach X.onion!'

In 'I cannot reach X.onion!', we should remove the phrase "... make sure that you have entered the 16-character.." as v2 onion services have been deprecated.

"If you cannot reach the onion service you desire, make sure that you have entered the 56-character onion address correctly; even a small mistake will stop Tor Browser from being able to reach the site....."