Deprecate AnyStr::chars_len again

I implemented 84e5f4212d6a5707748293b9603493661ac33355 again but I deprecate
the method again because it is too expensive.
See also https://github.com/RustPython/RustPython/pull/3164#discussion_r725438699

vm/src/anystr.rs

@@ -145,7 +145,9 @@ pub trait AnyStr<'s>: 's {
     // FIXME: get_chars is expensive for str
     fn get_chars(&self, range: std::ops::Range<usize>) -> &Self;
     fn bytes_len(&self) -> usize;
-    fn chars_len(&self) -> usize; // cannot access to cache here
+    // NOTE: str::chars().count() consumes the O(n) time. But pystr::char_len does cache.
+    //       So using chars_len directly is too expensive and the below method shouldn't be implemented.
+    // fn chars_len(&self) -> usize;
     fn is_empty(&self) -> bool;
 
     fn py_add(&self, other: &Self) -> Self::Container {

vm/src/builtins/pystr.rs

@@ -1645,10 +1645,6 @@ impl<'s> AnyStr<'s> for str {
         Self::len(self)
     }
 
-    fn chars_len(&self) -> usize {
-        self.chars().count()
-    }
-
     fn py_split_whitespace<F>(&self, maxsplit: isize, convert: F) -> Vec<PyObjectRef>
     where
         F: Fn(&Self) -> PyObjectRef,

vm/src/bytesinner.rs

@@ -1073,10 +1073,6 @@ impl<'s> AnyStr<'s> for [u8] {
         Self::len(self)
     }
 
-    fn chars_len(&self) -> usize {
-        Self::len(self)
-    }
-
     fn py_split_whitespace<F>(&self, maxsplit: isize, convert: F) -> Vec<PyObjectRef>
     where
         F: Fn(&Self) -> PyObjectRef,