Astropy: Fix or remove io.ascii C reader parallel code

[astrofrog]

As discussed in #8858, parallel reading with the io.ascii fast C reader is currently broken.

In https://github.com/astropy/astropy/pull/10880 I made the parallel reading option fall back to serial reading for now. I think that we should either fix this in a timely way or remove the parallel code to avoid code rot. I'm milestoning this as v4.2 but we could change to v4.3 if this is too aggressive.

cc @taldcroft @dhomeier @saimn

[pllim]

@taldcroft did mention that 4.2 is too aggressive over at https://github.com/astropy/astropy/pull/10880#issuecomment-710021187 , so I took the liberty to move the milestone. FYI