Remove reference to non-existent directory containing static files to fix warning when building developer documentation

[lukaszgo1]

Link to issue number:

None - related to #12971

Summary of the issue:

When building developer documentation the following warning is shown:

WARNING: html_static_path entry '_static' does not exist

Description of user facing changes

One less warning when building developer documentation is shown.

Description of development approach

Since we don't have any custom static files I've just removed the path - that was a left over from the default Sphinx template.

Testing strategy:

Created developer documentation made sure that this specific warning is gone.

Known issues with pull request:

None known

Change log entries:

None needed.

Code Review Checklist:

• Pull Request description:
  • description is up to date
  • change log entries
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• API is compatible with existing add-ons.
• Documentation:
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• Security precautions taken.