refactor: remove the mulled command and all related code

[Midnighter]

Based on Slack discussion, we decided to completely remove the mulled command and its dependencies.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #2199 (1bde222) into dev (4a29080) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #2199      +/-   ##
==========================================
+ Coverage   71.57%   71.65%   +0.07%     
==========================================
  Files          77       76       -1     
  Lines        8369     8315      -54     
==========================================
- Hits         5990     5958      -32     
+ Misses       2379     2357      -22     

Impacted Files	Coverage Δ
nf_core/__main__.py	59.80% <ø> (+0.98%)	⬆️
nf_core/modules/__init__.py	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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[mirpedrol]

Thanks!

[mirpedrol]

Should we also add a link to your app in the documentation?

[Midnighter]

If you like, we can surely do that.

[mirpedrol]

I have added the link to the documentation nf-core/website#1743 and will merge this if that's ok @Midnighter

[Midnighter]

Please go ahead, yes ☺️