Use non-monkey patch describe in generators.

In a concerted effort to keep monkey patching to a minimum. The default
Rails spec generators should not use the monkey patched version of
`describe`.

Always using only the non-monkey patched `RSpec.describe`, instead of
inspecting the configuration, has the benefits of:

- not requiring RSpec to be loaded when the generators are run by Rails
- not introducing extra logic and state to handle the different states
- Resolve #1048
Merge pull request #1052 from rspec/no-monkey-patch-generators

Use non-monkey patch describe in generators.