Remove some WebOb leftovers.

The deleted comments were talking about a swob EmptyResponse object,
which doesn't exist; it seems this was left over from the big WebOb
removal. Also, swob doesn't exhibit that behavior, so there's no point
in having this extra code path around.

Further, it seems like the hack was only needed with WebOb < 1.0.8. I
went back to Swift 1.4.8 (Essex) and compared it to master, and this
is the result:

DLO manifest HEAD
=================

Essex (ffadbc3)
---------------
Small #segments --> Content-Length: 12345
Large #segments --> Content-Length: 0
Large #segments sans shenanigans --> Content-Length: 0

master (f63b58f)
----------------
Small #segments --> Content-Length: 12345
Large #segments --> Content-Length: 0
Large #segments sans shenanigans --> Content-Length: 0

So, whatever WebOb wackiness this was intended to hack around, it
doesn't seem to have been here for a long, long time.

Change-Id: I7b717b1b36de1139cc5b76c166d1715dbc34b332
Merge "Remove some WebOb leftovers."