ch8: wraperror2: remove map_err() call

To match the book text (Listing 8.15, which is supposed to match
ch8/misc/wraperror2.rs, actually remove the map_err() calls that
are not supposed to be needed once the std::convert::From trait
is implemented for the wrapped types.

ch8/misc/wraperror2.rs

@@ -29,8 +29,8 @@ impl From<net::AddrParseError> for UpstreamError {
 }
 
 fn main() -> Result<(), UpstreamError> {
-    let _f = File::open("invisible.txt").map_err(UpstreamError::IO)?;
-    let _localhost = "::1".parse::<Ipv6Addr>().map_err(UpstreamError::Parsing)?;
+    let _f = File::open("invisible.txt")?;
+    let _localhost = "::1".parse::<Ipv6Addr>()?;
 
     Ok(())
-}
+}