Drop the deprecated convert command

[codebien]

What?

It drops the har converter as we decided in #1124, and it is following the deprecation process started by #2714.

https://github.com/grafana/har-to-k6 is the alternative

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (8457c91) 72.95% compared to head (a56a0ae) 73.05%.

❗ Current head a56a0ae differs from pull request most recent head 1870bb1. Consider uploading reports for the commit 1870bb1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3365      +/-   ##
==========================================
+ Coverage   72.95%   73.05%   +0.09%     
==========================================
  Files         261      256       -5     
  Lines       20018    19642     -376     
==========================================
- Hits        14605    14349     -256     
+ Misses       4486     4406      -80     
+ Partials      927      887      -40     

Flag	Coverage Δ
ubuntu	73.05% <100.00%> (+0.15%)	⬆️
windows	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cmd/root.go	94.26% <100.00%> (ø)

... and 7 files with indirect coverage changes

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