Stop including `RSpec::Mocks::ExampleMethods` in the singleton class of the example

This is a followup to rspec/rspec-core#1188. It fixes #102.  (Actually, that was already fixed by those rspec-core changes, I discovered when porting @michihuber's specs from #238).

With this change, the user (or test framework integration gem, or whatever) is responsible for including `RSpec::Mocks::ExampleMethods` now that `RSpec::Mocks.setup` no longer does that.  I plan to add a deprecation warning to 2.99 for that.

One slightly odd thing is that `RSpec::Mocks.setup` is now a no-op.  I think @samphippen will use it for #449 (and rspec-core still calls it), so I kept it for now.  I wasn't sure if the "outside rspec" cukes should call it since it's not strictly needed at this point.  I guess we can address that in #449.

@samphippen -- the oddness you saw in #449 should be resolved now.