Removes 'fast read' plumbing.

- The dd info page mentions a special fast-read framework if no conv=FLAG is
  specified (see bs=N) which I left space for. As it turns out, this is performed already
  so it does not need to be implemented.

src/uu/dd/src/dd.rs

@@ -843,17 +843,6 @@ fn read_helper<R: Read, W: Write>(i: &mut Input<R>, o: &mut Output<W>, bsize: us
 {
     // Local Predicate Fns -----------------------------------------------
     #[inline]
-    fn is_fast_read<R: Read, W: Write>(i: &Input<R>, o: &Output<W>) -> bool
-    {
-        // TODO: Enable this once fast_reads are implemented
-        false &&
-        i.ibs == o.obs
-            && !is_conv(i)
-            && !is_block(i)
-            && !is_unblock(i)
-            && !i.cflags.swab
-    }
-    #[inline]
     fn is_conv<R: Read>(i: &Input<R>) -> bool
     {
         i.cflags.ctable.is_some()
@@ -882,15 +871,6 @@ fn read_helper<R: Read, W: Write>(i: &mut Input<R>, o: &mut Output<W>, bsize: us
         }
     }
    // ------------------------------------------------------------------
-    if is_fast_read(&i, &o)
-    {
-        // TODO: fast reads are copies performed
-        // directly to output (without creating any buffers)
-        // as mentioned in the dd spec.
-        unimplemented!()
-    }
-    else
-    {
    // Read
    let mut buf = vec![BUF_INIT_BYTE; bsize];
    let mut rstat = match i.cflags.sync
@@ -920,7 +900,6 @@ fn read_helper<R: Read, W: Write>(i: &mut Input<R>, o: &mut Output<W>, bsize: us
    {
        Ok((rstat, buf))
    }
-    }
 }
 
 fn print_io_lines(update: &ProgUpdate)