Revert "Omnibus to implement CFRunLoop with kevent."

[compnerd]

Reverts #3004

The change seems to have caused a regression on Windows between 9bc1a18...5f57e70

[compnerd]

@swift-ci please test

[3405691582]

I'd suggest seeing if setting __CFNanosecondsToTSR to just return ns solves the regression on Windows, as the base code is problematic as written anyway (see #4220).

[3405691582]

Actually, since __CFTSRRate = 1.0e7 on Windows, it probably should be something like:

static inline uint64_t __CFNanosecondsToTSR(uint64_t ns) {
#if TARGET_OS_MAC || TARGET_OS_LINUX
    return ns;
#elif TARGET_OS_WINDOWS
    CFTimeInterval ti = ns / 1.0E7;
    return __CFTimeIntervalToTSR(ti);
#else
    CFTimeInterval ti = ns / 1.0E9;
    return __CFTimeIntervalToTSR(ti);
#endif
}