[LTP] RFC remove README.kernel_config

[xuyang2018.jy@fujitsu.com]

Hi Petr
I am fine with removing it.
> Hi,
>
> does anybody use README.kernel_config? Is it useful for anybody?
> There is a patch [1] which enhances it, but I'd prefer to delete it and replaces
> the content with .needs_kconfigs setup [2].
>
> Kind regards,
> Petr
>
> [1] https://patchwork.ozlabs.org/project/ltp/patch/20210302073248.10204-1-pankaj.joshi347@gmail.com/
> [2] https://github.com/linux-test-project/ltp/wiki/Test-Writing-Guidelines#2228-parsing-kernel-config
>