docs: remove `pip install` from TensorBoard.dev guide (#3473)
docs: remove `pip install` from TensorBoard.dev guide (#3473)

Summary:
The Colab image has the latest version of TensorBoard (2.2.0) bundled,
so there’s no need to go through a separate install.

Test Plan:
Clicked “Run all” in Colab, and the whole notebook worked as designed.

wchargin-branch: docs-tbdev-no-pip-install
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard