core/char: Drop `radix == 10` special case

This seems to perform equally well

src/libcore/char/methods.rs

@@ -122,14 +122,7 @@ pub fn is_digit(self, radix: u32) -> bool {
     #[inline]
     pub fn to_digit(self, radix: u32) -> Option<u32> {
         assert!(radix <= 36, "to_digit: radix is too high (maximum 36)");
-        if radix == 10 {
-            return match self {
-                '0' ..= '9' => Some(self as u32 - '0' as u32),
-                _ => None,
-            };
-        }
-
-        let val = if radix < 10  {
+        let val = if radix <= 10  {
             match self {
                 '0' ..= '9' => self as u32 - '0' as u32,
                 _ => return None,