Remove Symbol::len

It is used exactly once and can be replaced with the equally fast
.as_str().len()

compiler/rustc_expand/src/proc_macro_server.rs

@@ -577,7 +577,7 @@ fn from_str(&mut self, s: &str) -> Result<Self::Literal, ()> {
             }
 
             // Synthesize a new symbol that includes the minus sign.
-            let symbol = Symbol::intern(&s[..1 + lit.symbol.len()]);
+            let symbol = Symbol::intern(&s[..1 + lit.symbol.as_str().len()]);
             lit = token::Lit::new(lit.kind, symbol, lit.suffix);
         }
 

compiler/rustc_span/src/symbol.rs

@@ -1642,10 +1642,6 @@ pub fn as_u32(self) -> u32 {
         self.0.as_u32()
     }
 
-    pub fn len(self) -> usize {
-        with_session_globals(|session_globals| session_globals.symbol_interner.get(self).len())
-    }
-
     pub fn is_empty(self) -> bool {
         self == kw::Empty
     }