rm nexus sdk usage and dependency

[ssssarah]

I've put all the code from the sdk that was used in the file previous_sdk_usage, with the change that the endpoint and token are provided as parameters (as opposed to setting environment variables)

No design thought really went into this, it's a fast solution, up to change

(preparatory work so that we specify a token as a callable, without having to handle setting the nexussdk environment variable)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 24.63768% with 104 lines in your changes are missing coverage. Please review.

Project coverage is 74.76%. Comparing base (d2604cc) to head (b3c1540).

Files	Patch %	Lines
...forge/specializations/stores/nexus/http_helpers.py	21.49%	84 Missing :warning:
utils.py	16.66%	20 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #407 +/- ## ========================================== - Coverage 75.61% 74.76% -0.86% ========================================== Files 103 104 +1 Lines 6779 6883 +104 ========================================== + Hits 5126 5146 +20 - Misses 1653 1737 +84 ``` | [Flag](https://app.codecov.io/gh/BlueBrain/nexus-forge/pull/407/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/BlueBrain/nexus-forge/pull/407/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | `74.76% <24.63%> (-0.86%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain#carryforward-flags-in-the-pull-request-comment) to find out more.

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[crisely09]

Hmm why to call things: previous_sdk_usage.py ? why not just something generic about the methods there? and just comment/docstring that this was previois sdk usage. May be one can call it https_helpers.py or something on those lines, it's just ... the name is too specific, "previous" and "sdk" ... I dunno ...