remove readtable, writetable and related macros

[bkamins]

Fixes #1602

I propose to finally remove readtable and writetable and related macros from deprecated.
I have checked that this reduces load time of DataFrames.jl about 0.15 seconds.
The functions are long deprecated and not maintained. I think CSV.jl is now mature-enough to be considered as a full replacement (after the recent fix of Tabler.rows all performance issues with CSV.jl are resolved AFAICT).

[quinnj]

Yeah, I think we're in a good spot to do this. Getting a 0.15s speed up in package loading is great! Happy to help manage anything that comes up on the CSV.jl side for this.

[bkamins]

You have been managing CSV.jl for years I would say 😄. Actually my crucial point is that no one knows now if readtable and writetable actually work correctly in all cases as they have not been maintained since #1243, which is ages for DataFrames.jl.

[nalimilan]

OK, I guess they have been deprecated for long enough, and 0.15s is a nice speedup!

[bkamins]

OK - thank you. Let us get rolling with this.

@quinnj - from this moment CSV.jl is the king (you can count on my support there, as likely we are going to keep getting issues there, like the one I have just opened in JuliaData/CSV.jl#558).