Remove fivemat scenario.

We can't support five mat since it's so non-standard and has some hacks.

- `Fivemat.new` doesn't return an instance of `Fivemat` since it's a module:

https://github.com/tpope/fivemat/blob/5586092ae26241484f5e71fd27de350dfff0a1fa/lib/fivemat.rb#L3-L19

- It rebinds a method from the super-superclass: 

https://github.com/tpope/fivemat/blob/5586092ae26241484f5e71fd27de350dfff0a1fa/lib/fivemat/rspec.rb#L61-L64