Remove all istanbul-related dependencies

[Istanbul](https://istanbul.js.org)-related dependencies were
previously added (some years ago) to provide coverage report for our
integration and memory tests.

However, I decided to remove all of them for the following reasons (from
the most to the least important):

  - Coverage reports were never really generated for those kind of
    tests, they were only really used for unit tests, were we rely on
    another tool (I think Jest embarks some kind of coverage logic or
    dependency)

  - Installing all dependencies without our package-lock led to an
    unsolvable dependency tree (I don't really now why it didn't with
    our package-lock though). Removing those dependencies seems to remove
    this problem

  - There were security alerts that were shouted by npm and GitHub due to
    them.
    Even if that was probably nothing serious, I dislike seeing all this
    red and having to think about it.

  - Those packages' last version were kind of old, so maybe that's not
    what the cool new kids are using nowadays - Uhm I'm mean, there
    might be better way to do those things since then, if we ever need to
    integrate those reports again

  - It removes dependencies and I don't like too much dependencies,
    it makes me realize that there's too many things going on that I have
    no idea about!

  - Doing more commits and PRs improves my GitHub stats/cred.

  - There's other cities this dependency could be named after.
    If that's a pun however, I don't get it and that makes me feel dumb.

  - Why not?