Removed ResourceTiming Level2 document

We had a Level 2 document that was a cut and paste from Level 1 and not
kept up to date as we made changes to Level 1.

Ilya made some changes to Level 2 doc in the last few weeks to add 4 new
attributes to ResourceTiming: nextHopProtocol, transferSize,
decodedBodySize and encodedBodySize). I incorporated those into the Level 1
doc, and now we only have one document for Resource Timing which is at:
http://w3c.github.io/resource-timing/

Nothing has changed as such. This is just a cleanup as we never really
worked on Level 2 document since work on Level 1 has continued till date.

Arvind