chore(settings): deprecate and remove module based settings

[jansegre]

Motivation

Using HATHOR_CONFIG_FILE has been deprecated in favor of HATHOR_CONFIG_YAML for a while now. I feel like it's time to remove it.

Acceptance Criteria

• Log a "critical" error when HATHOR_CONFIG_FILE is used and fail loading the settings (this will fail very early on when running any command);
• Clean up hathor/conf/get_settings.py now that it only has to load YAML settings;
• Remove hathor/conf/mainnet.py, hathor/conf/testnet.py and hathor/conf/unittests.py;
• Remove module settings tests from tests/others/test_hathor_settings.py;
• Fix tests/resources/p2p/test_status.py (it was still using module settings to get genesis hashes).

Checklist

• [x] If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (8b13de7) 85.32% compared to head (dbc7a21) 85.43%.

Files	Patch %	Lines
hathor/conf/get_settings.py	40.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #942      +/-   ##
==========================================
+ Coverage   85.32%   85.43%   +0.11%     
==========================================
  Files         290      287       -3     
  Lines       22477    22450      -27     
  Branches     3380     3379       -1     
==========================================
+ Hits        19179    19181       +2     
+ Misses       2626     2606      -20     
+ Partials      672      663       -9     

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