EEPROM refactor: remove eeprom_teensy.c by default, use transient instead

[tzarc]

Description

Default implementation of EEPROM when unspecified was to use a transient-like implementation, implemented in eeprom_teensy.c. This moves the normal implementation to be the actual transient EEPROM implementation, decoupling the implementations further.

All keyboards build, apart from the same two boards currently failing on develop.

Types of Changes

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

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).