rockpi-s: Remove legacy kernel 4.4 support

[ColorfulRhino]

Description

The legacy U-Boot can't be built anymore and 4.4 is EOL.
As discussed in #6821 (comment)

I didn't see any kernel or uboot patch folder. If I forgot something related to legacy, please let me know :)

GitHub issue reference: #6821
Jira reference number AR-2392

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

[efectn]

Why don't we just migrate rockpi-s into the rockchip64 and move family config to board config?

[ColorfulRhino]

Why don't we just migrate rockpi-s into the rockchip64 and move family config to board config?

Yeah this was the plan: #6821 (comment)

There are only a few board families remaining that are only used by one board. rockpis and rock-s0 are 2 of them and can be integrated into rockchip64.