Remove invalid keyboard_folder from gk6

[zvecr]

Description

Currently producing:

☒ gizmo_engineering/gk6: Invalid matrix config.

Causes https://github.com/qmk/qmk_firmware/blob/master/lib/python/qmk/info.py#L406 to fail to find config.h files. If absent, it is calculated at build time.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).