Remove implicit lab controller XML-RPC retries

Several of the calls from the lab controllers back to the main
server aren't idempotent, so retrying isn't safe in general.
The way kobo does retries is also impossible to debug (it writes
to sys.stderr instead of using the logging subsystem, which
means the Beaker lab controller logs don't capture the info)

Dropping the retries entirely will help to diagnose network
stability issues, and will also allow retries to be selectively
reintroduced where they make sense.

Bug: 974352
Change-Id: I33ee709a959707d23125898207101e5c8ee96124