Remove unused methods on the private Wtf8 type

The type and its direct parent module are `pub`, but they’re not reachable outside of std

Author: SimonSapin

src/libstd/sys_common/wtf8.rs

@@ -875,24 +875,6 @@ impl Hash for Wtf8 {
     }
 }
 
-impl Wtf8 {
-    pub fn is_ascii(&self) -> bool {
-        self.bytes.is_ascii()
-    }
-    pub fn to_ascii_uppercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_uppercase() }
-    }
-    pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
-        Wtf8Buf { bytes: self.bytes.to_ascii_lowercase() }
-    }
-    pub fn eq_ignore_ascii_case(&self, other: &Wtf8) -> bool {
-        self.bytes.eq_ignore_ascii_case(&other.bytes)
-    }
-
-    pub fn make_ascii_uppercase(&mut self) { self.bytes.make_ascii_uppercase() }
-    pub fn make_ascii_lowercase(&mut self) { self.bytes.make_ascii_lowercase() }
-}
-
 #[cfg(test)]
 mod tests {
     use borrow::Cow;