remove mocking framework 'mocha'

This PR removes `mocha` as a dependency from OpenProject (Core).

Numerous times I got confused when using `stub`, `stubs`, `returns`, `and_return`, `expects`, `should_receive` etc. due to rspec and mocha providing mocking facilities. Today, I ran into some trouble where a mocha mock wasn't removed correctly. Since nowadays `rspec-mocks` provides almost the same mocking capabilities as mocha does, I decided to remove the last remaining bits of it and exclusively use rspec-mocks in OpenProject's core.

note to reviewer: second commit does indentation only.