Remove insignificant notes from CStr documentation

These notes are about a distinction that is not going to be observable
in the API. Whether or not the UTF-8 check knows the string length ahead
of time, these methods require linear time.

src/libstd/ffi/c_str.rs

@@ -1187,11 +1187,6 @@ impl CStr {
     /// function will return the corresponding [`&str`] slice. Otherwise,
     /// it will return an error with details of where UTF-8 validation failed.
     ///
-    /// > **Note**: This method is currently implemented to check for validity
-    /// > after a constant-time cast, but it is planned to alter its definition
-    /// > in the future to perform the length calculation in addition to the
-    /// > UTF-8 check whenever this method is called.
-    ///
     /// [`&str`]: ../primitive.str.html
     ///
     /// # Examples
@@ -1220,11 +1215,6 @@ impl CStr {
     /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD] and return a
     /// [`Cow`]`::`[`Owned`]`(`[`String`]`)` with the result.
     ///
-    /// > **Note**: This method is currently implemented to check for validity
-    /// > after a constant-time cast, but it is planned to alter its definition
-    /// > in the future to perform the length calculation in addition to the
-    /// > UTF-8 check whenever this method is called.
-    ///
     /// [`Cow`]: ../borrow/enum.Cow.html
     /// [`Borrowed`]: ../borrow/enum.Cow.html#variant.Borrowed
     /// [`Owned`]: ../borrow/enum.Cow.html#variant.Owned