Revert "threads: yield on get if we had run for too long."
Revert "threads: yield on get if we had run for too long."

This reverts commit bca11ae749877663d9d0b3fab389f44bacd01474.

Yielding in future::get() may cause unexpected and very surprising side
affect to happen: a function that suppose to be atomic may not be.
Consider the example here:

static bool x;

void atomic() {
  x = true;
  function_returning_future().then_wrapped([] (auto&& f) {
     f.get();
  });
  assert(x);
  x = false;
}

Can assert() here ever trigger? It turns out that if atomic() is called
from a seastar thread context and function_returning_future() returns
ready future f.get() may yield and other call to atomic() may reset x to
false and cause the assert to trigger.

Message-Id: <cbb3256a3ef61f4e72dccd00e572d496d1fb7db4@scylladb.com>

dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar