Remove Error::cause impls equal to deafult impl

Author: faern

src/libstd/sync/mpsc/mod.rs

@@ -1581,10 +1581,6 @@ impl<T: Send> error::Error for SendError<T> {
     fn description(&self) -> &str {
         "sending on a closed channel"
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1624,10 +1620,6 @@ impl<T: Send> error::Error for TrySendError<T> {
             }
         }
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
@@ -1652,10 +1644,6 @@ impl error::Error for RecvError {
     fn description(&self) -> &str {
         "receiving on a closed channel"
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1685,10 +1673,6 @@ impl error::Error for TryRecvError {
             }
         }
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]
@@ -1726,10 +1710,6 @@ impl error::Error for RecvTimeoutError {
             }
         }
     }
-
-    fn cause(&self) -> Option<&dyn error::Error> {
-        None
-    }
 }
 
 #[stable(feature = "mpsc_error_conversions", since = "1.24.0")]