Remove unnecessary impl std::error::Error

Author: olehmisar

src/error/multiple_error_types/define_error_type.md

@@ -16,7 +16,6 @@ Rust allows us to define our own error types. In general, a "good" error type:
 * Composes well with other errors
 
 ```rust,editable
-use std::error;
 use std::fmt;
 
 type Result<T> = std::result::Result<T, DoubleError>;
@@ -38,14 +37,6 @@ impl fmt::Display for DoubleError {
     }
 }
 
-// This is important for other errors to wrap this one.
-impl error::Error for DoubleError {
-    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
-        // Generic error, underlying cause isn't tracked.
-        None
-    }
-}
-
 fn double_first(vec: Vec<&str>) -> Result<i32> {
     vec.first()
         // Change the error to our new type.