Remove the "Experimental additional constraints" from Readable

[CaseyCarter]

From #183:

We can probably just remove these sanity checks from range-v3, cmcstl2, and the Ranges TS. I added them to keep myself honest when I was designing and implementing proxy iterator support. Now they only serve to make compile times worse. Holler if you see a reason to keep them.

I see no reason to keep them; we should strike those requirements from P0022 before moving it into the TS.

Proposed Resolution:

Wording relative to P0022R2. Change the definition of the Readable concept in [iterators.readable] as follows:

template <class I>
concept bool Readable() {
  return Movable<I>() && DefaultConstructible<I>() &&
    requires (const I& i) {
      typename value_type_t<I>;
      typename reference_t<I>;
      typename rvalue_reference_t<I>;
      { *i } -> Same<reference_t<I>>;
      { iter_move(i) } -> Same<rvalue_reference_t<I>>;
    } &&
    // Relationships between associated types
    CommonReference<reference_t<I>, value_type_t<I>&>() &&
    CommonReference<reference_t<I>, rvalue_reference_t<I>>() &&
+    CommonReference<rvalue_reference_t<I>, const value_type_t<I>&>();
-    CommonReference<rvalue_reference_t<I>, const value_type_t<I>&>() &&
-    Same<
-      std::common_reference_t<reference_t<I>, value_type_t<I>>,
-      value_type_t<I>>() &&
-    Same<
-      std::common_reference_t<rvalue_reference_t<I>, value_type_t<I>>,
-      value_type_t<I>>();
}