Remove From trait impl for ExternalSyntaxContext, privatize

`is_root`

Author: aerooneqq

compiler/rustc_span/src/hygiene.rs

@@ -1484,18 +1484,11 @@ rustc_index::newtype_index! {
 
 impl ExternalSyntaxContext {
     #[inline(always)]
-    pub fn is_root(self) -> bool {
+    fn is_root(self) -> bool {
         self.as_u32() == 0
     }
 }
 
-impl From<SyntaxContext> for ExternalSyntaxContext {
-    #[inline]
-    fn from(ctxt: SyntaxContext) -> Self {
-        ExternalSyntaxContext::from_u32(ctxt.as_u32())
-    }
-}
-
 // Decodes `SyntaxContext`, using the provided `HygieneDecodeContext`
 // to track which `SyntaxContext`s we have already decoded.
 // The provided closure will be invoked to deserialize a `SyntaxContextData`