native: Remove timerfd implementation on linux

Rust advertises itself as being compatible with linux 2.6.18, but the timerfd
set of syscalls weren't added until linux 2.6.25. There is no real need for a
specialized timer implementation beyond being a "little more accurate", but the
select() implementation will suffice for now.

If it is later deemed that an accurate timerfd implementation is needed, it can
be added then through some method which will allow the standard distribution to
continue to be compatible with 2.6.18

Closes #13447
auto merge of #13455 : alexcrichton/rust/jettison-timerfd, r=brson

Rust advertises itself as being compatible with linux 2.6.18, but the timerfd
set of syscalls weren't added until linux 2.6.25. There is no real need for a
specialized timer implementation beyond being a "little more accurate", but the
select() implementation will suffice for now.

If it is later deemed that an accurate timerfd implementation is needed, it can
be added then through some method which will allow the standard distribution to
continue to be compatible with 2.6.18

Closes #13447