Remove the unspecified check about .get(exhausted_range_inclusive)

`RangeInclusive::is_empty` (rust 1.47+) is in conflict with our MSRV (1.43.1) so we can't use it yet.
The unspecified behavior is documented in `Itertools::get` so we simply remove this.

src/iter_index.rs

@@ -52,7 +52,6 @@ where
     type Output = Take<Skip<I>>;
 
     fn index(self, iter: I) -> Self::Output {
-        debug_assert!(!self.is_empty(), "The given `RangeInclusive` is exhausted. The result of indexing with an exhausted `RangeInclusive` is unspecified.");
         iter.skip(*self.start())
             .take((1 + *self.end()).saturating_sub(*self.start()))
     }