Retire git tf/git-tfs content

[HonkingGoose]

Introduction:

It's been a while since we asked if anyone has any objections to removing the TFS content:

In #1322 @HonkingGoose recommended removing the git-tf content, because git-tf is now EOL. Is anyone interested in keeping it?

Since October 2019 nobody complained about the proposed removal, so I will put my head on the line, and remove the TF/TFS sections and references entirely. If we do get complaints, we can always do a git revert on the merge commit and return to the original state.

Make a build, check my work:

I cannot make a build locally, due to issues with installing a working set of dependencies.
Please run a build on your end, and check my work.

Changes:

• Remove the client-tfs and import-tfs files.
• Manually edited the JSON file to remove references to the removed files.
• Searched and removed matches on text string tf and tfs in the book text.
• Remove the git-tfs checkin tool image

Issues closed by merging this pull-request:

Closes #1326
Closes #1323

Action to take by @HonkingGoose after merge:

Open PR at https://github.com/git-tfs/git-tfs#more-advanced-use-cases to remove the reference that's there to our just deleted sections of the book. EDIT: made a pull request for this, see below.

Discussion:

Hi @ben, I hope you like this, feel free to make edits within this pull-request as needed. I won't be force pushing anything on this branch.

[ben]

Let's do it!