helper: remove timings["existence_check"], aka "Already-In-Grid Check"

helper: remove timings["existence_check"], aka "Already-In-Grid Check"

This measured how long the Helper took to do a filecheck before asking
for ciphertext. The "Contacting Helper" report includes both
existence_check and the client-helper RTT.

For non-overlapping uploads, it was being returned correctly. But when
multiple upload requests overlapped, and the file was not already in the
grid, the filecheck would only run once, and its existence_check time
would be reported for all uploaders (even if they didn't have to wait
for that time). Cleaning that up proved too difficult: the only correct
place to report this time is from the initial remote_upload_chk() call,
but the return value of that is too constrained to accomodate it in the
needs-upload case.

So I'm removing it altogether. Eventually I plan to add a proper
events/times field and record more data, including this check, in a form
that can be drawn on a nice zoomable timeline view.

Old clients talking to a new Helper (which doesn't supply the value)
will tolerate the loss (they'll just display an empty field on the web
view).