[R] Remove usage of cpp11's cpp11/altrep.hpp and HAS_ALTREP

[DavisVaughan]

Describe the enhancement requested

cpp11 is moving to require R >=4.0.0 in the next release r-lib/cpp11#411

That means that ALTREP is always available, and the macro HAS_ALTREP is no longer needed, and the file cpp11/altrep.hpp is no longer useful. I'm going to keep them around for a release or two, but if you could remove your reliance on them, then that'd be great!

https://github.com/search?q=repo%3Aapache%2Farrow%20altrep.hpp&type=code

Component(s)

R