Remove realtime kernels

I'm looking into cleaning up the kernel build and the number of kernels build. The last update to the -rt kernels was with:

6f40ada81cb6 ("enable rt for 5.4.x")

on 2020-04-08 and we had to disable -rt kernel builds with:

c1f4c392cc34 ("kernel: Disable -rt kernel temporarily")

on 020-05-09 since the patches did not apply cleanly anymore to the updated kernels.

I'm not aware of any users of the -rt kernels and would like to remove support in about a month time.

If anyone is relying on them and would like to maintain please let us know.

@TiejunChina you added the initial support and maintained the patches since. What's your view?

@rn I can keep working on this. But I'm so busy this month. Let me find some time to jump back next month. Can this work for you?

@TiejunChina thanks for the quick response. Happy to keep the -rt support since i know you'll update it soon.