Deprecate and remove TestUtils.mockComponent

[gaearon]

It is a Jest-ism that shouldn't be in our open source package.
People using Jest can implement a similar pattern directly.

[gaearon]

Test Utils, not documentation 😛
Sorry the colors were the same.

[bvaughn]

Oh whoops! My apologies.

[chrisdieckhaus]

Does this issue still need to be taken care of? If so, I'll take a stab at it.