Remove unneeded attribute.

Author: reitermarkus

library/core/src/ffi/c_str.rs

@@ -250,7 +250,6 @@ impl CStr {
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_cstr_methods", issue = "101719")]
-    #[rustc_allow_const_fn_unstable(const_eval_select)]
     pub const unsafe fn from_ptr<'a>(ptr: *const c_char) -> &'a CStr {
         // SAFETY: The caller has provided a pointer that points to a valid C
         // string with a NUL terminator of size less than `isize::MAX`, whose