Remove unnecessary cleanup of AXParameter callbacks

[killenheladagen]

There is no need to unregister AXParameter callbacks. The hash table used for storing them will free its contents in ax_parameter_free().

Checklist before requesting a review

• [x] I have performed a self-review of my own code
• [x] I have verified that the code builds perfectly fine on my local system
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have verified that my code follows the style already available in the repository
• [ ] I have made corresponding changes to the documentation