feat(protractor): add removeMockModule method to protractor.

This fixes the lack of control in removing individual mock modules.

In the past there were only two ways to get rid of a mock module:
1. Overriding the mock module with an empty one.
2. Clearing all mock modules with `browser.clearMockModules()`

Method 1 is a very ugly hack to achieve this goal and method 2 removes all mock
modules modules including the ones we might want to keep.

This pullrequests adds a new method `removeMockModule(name);` that does the
opposite of the `addMockModule(name)` method.