Helix/pico move to split_common

[mtei]

Description

• Abolished the custom matrix.c and split_util.c of helix/pico and changed to use split_common.
• Organized helix/pico/post_rules.mk to minimize support with the HELIX = option.
• Removed unused OLED code.

@MakotoKurauchi, This PR needs your review and approval.

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

• Change the Helix keyboard to use split_common. #16388

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).

[zvecr]

@Biacco42 sorry this was missed, however you should have been pinged for review. Let us know if there are any issues with the change, and if you need any assistance on any merge rebase issues that have come from it.