Remove unused fake run_instance() method

In a bunch of tests, we stub out rpc.call() and use it as a hook to
create instances and return their details.

The thing is, we never actually call() 'run_instance anymore .. we
always cast() it. So, these fake methods never actually get called and
can be safely removed.

A similar change was made in I63bbc98c285faebec53f8e62857c01548807db68
but some more copies of this have been added recently. Copy and pasted
code FTW!

blueprint: oslo-messaging
Change-Id: I1a5a048ed9b0d939d50f7728a348bb6c5a6074dc
Merge "Remove unused fake run_instance() method"