Remove BufMut::bytes_vectored_mut() (#430)

There are issues with regard to uninitialized memory. We are avoiding
stabilizing this function for now.

src/buf/ext/chain.rs

@@ -3,8 +3,6 @@ use crate::{Buf, BufMut};
 
 use core::mem::MaybeUninit;
 
-#[cfg(feature = "std")]
-use crate::buf::IoSliceMut;
 #[cfg(feature = "std")]
 use std::io::IoSlice;
 
@@ -210,13 +208,6 @@ where
 
         self.b.advance_mut(cnt);
     }
-
-    #[cfg(feature = "std")]
-    fn bytes_vectored_mut<'a>(&'a mut self, dst: &mut [IoSliceMut<'a>]) -> usize {
-        let mut n = self.a.bytes_vectored_mut(dst);
-        n += self.b.bytes_vectored_mut(&mut dst[n..]);
-        n
-    }
 }
 
 impl<T, U> IntoIterator for Chain<T, U>