Deprecate/Remove `app` query parameter and the NBGITPULLER_APP default value

We have a `app` query parameter that is accepted in the `/git-pull` endpoint that influences the redirection path to unless `urlpath` is also set. A default value for this query parameter can be set via `NBGITPULLER_APP`.

I'm considering if we should remove it or attempt to deprecate it etc to reduce the complexity.

# Background

## @manics wrote

> [...] there's an undocumented environment variable `NBGITPULLER_APP`
https://github.com/jupyterhub/nbgitpuller/blob/fdd54bdd2b9c6452dcbeff8acf07a5accf7b1b64/nbgitpuller/handlers.py#L149
Do you know it's significance?

## I replied

>@manics it seem to have been added by me four years ago in https://github.com/jupyterhub/nbgitpuller/pull/41 :open_mouth:
>
>I did some git history inspection:
>
>- #41 added NBGITPULLER_APP, a default value for an `app` parameter accepting `notebook` and `lab`.
>- Documentation about it updated in #48
>- Documentation on constructing the url removed in favor of using a link generator in #62
>
>Overall, it seems that it does the single thing of "if set to lab", the web handler for `/git-pull` endpoint will default to prepending `/lab/tree` to a post git-pull redirection path. It won't affect `gitpuller` the CLI though.
>
>I conclude that the https://jupyterhub.github.io/nbgitpuller/link only crafts links using `urlpath` directly though, and the `NBGITPULLER_APP` is a default value for a `app` query parameter, which only has an influence if `urlpath` isn't set.
>
>I'd love to see this env var and the entire `app` query parameter removed to reduce complexity, but breaking existing links isn't fun. @yuvipanda do you have a suggestion with regards to something to do with the logic about having an `app` parameter and/or the NBGITPULLER_APP env which is the `app` parameters default value?