Remove Tag from TreeEntryTargetType enum

[martinwoodward]

Not completely sure if we should just remove this or if @nulltoken was trolling me when we said he wanted it obsoleted. Obsoleting for now, we can just remove later.

[nulltoken]

How about changing the deprecation message into "This entry will be removed in the next release as it is not a valid TreeEntryTargetType."?

[martinwoodward]

My implant makes kicks in when I try to promise to do things in the future that I can't guarantee ;-) Said we'll be removing it "soon", how about that?

[dahlbyk]

Compromise: including the removal as a separate commit in this PR so @nulltoken can merge all but the removal for now, and close the PR completely after the next release.

[nulltoken]

Said we'll be removing it "soon", how about that?
Well, "in the next release" is pretty standard pattern. And we always held to our promises ;-) See #181 and #400.

The plan is to release this PR in v0.11.1 (as soon as #430 is merged).
Once v0.11.1 is tagged and published, we drop the [Obsolete] decoration. When v0.12 is released, the enum entry is already dropped.

[nulltoken]

💖

Now, if you feel like it you can initiate another pull request to get rid of the deprecated entry.