[rfa:symtab] Delete broken "memory mapped objfile cache"

[Andrew Cagney]

> Actually, apple has it implemented and working.
> Maybe they'd like to share?
> If so, and they need that code, maybe we should keep it around for just a bit longer.

(I've this strange feeling of dejavu ...)

As I stated, I think it is better to clear the decks so that Apple (or 
who ever) are free to add a clean and robust implementation of the 
technique.

Andrew