Remove redundant tag/create API endpoint

[sneakypete81]

This endpoint is not used and has been removed from the frontend development branch. See #34 comments for more details.

[sneakypete81]

@jmathai: what about tag/delete, does this actually remove the tag from its associated photos?
If not, I suggest we remove this endpoint too, to avoid confusion.

[sneakypete81]

The tag/create endpoint hasn't actually been removed yet - see #34.