Add way of removing mock functions from schema

[roninjin10]

When using executable schema, instantiating a new Apollo client can be slow. To speed it up it would be helpful to be able to be able to do the reverse of addMockFunctionsToSchema and remove the mocks in between tests.

For our tests we use buildClientSchema and makeExecutableSchema to mock out our entire graphql schema automatically based on type. Then we provide only the mocks relavent to the test with addMockFunctionsToSchema. This way of mocking is extremely convenient but is very slow because of needing to instantiate a new executable schema in every test. I attempted to add a removeMocksFromSchema myself but this turned out to be very difficult.