Remove deprecated cli commands and --migrate-pytest flag with all obsoleted code.

[JulianFlesch]

Changes:

• Remove previously deprecated cli commands
• Remove pytest linting / migration:
  • Remove --migrate-pytest flag from cli
  • Remove migrate_pytest attribute from ComponentCreate and subclasses (SubworkflowCreate and ModuleCreate)
  • Remove related tests

closes #4160

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.16%. Comparing base (854fdc9) to head (b9af28b).
⚠️ Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
nf_core/__main__.py	50.00%	3 Missing ⚠️
nf_core/commands_subworkflows.py	66.66%	1 Missing ⚠️

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[mashehu]

@nf-core-bot changelog