Remove BytesStart::unescaped* set of methods because they could return wrong results

[Mingun]

• Closes BytesStart unescaped method returns the attributes in the same result #118
• Closes Unescaped now only returns the tag name #119
• Closes Deprecate unnecessary unescaping of tags #397

I decide to completely remove this set of methods because they could produce misleading results.

I decide to remove example issue68.rs because of reasons:

• it used removed method
• it contained a bug, described in Fix caldav example #396
• it wasn't a good example at all because of unused code and lacking of comments, instead of fixing that I think it is better to remove it completely, working with namespaces will changed soon anyway

[dralley]

@Mingun FYI, the benches are broken again