Remove deprecated mocha/setup.rb mechanism

Previously this was the recommended way to integrate Mocha with a test
framework. However, this mechanism was deprecated in #399 in favour of
requiring a file specific to the relevant test framework, i.e.
mocha/test_unit.rb or mocha/minitest.rb. Test frameworks like RSpec
integrate by requiring mocha/api.rb.

This old mechanism was deprecated in v1.10.0 [1,2] and so is safe to
remove now.

[1]: 36adf88
[2]: 388f44d