Revert: Make Option.flatten const

Option.flatten relies on Option.and_then, which has a generic trait
bound and can't be const yet. From @oli-obk:

> please undo all constifications for functions that take generic things
> with trait bounds (other than Sized), these require
> rust-lang/rfcs#2632 first

Author: 9999years

src/libcore/option.rs

@@ -1742,16 +1742,6 @@ impl<T> Option<Option<T>> {
     /// ```
     #[inline]
     #[stable(feature = "option_flattening", since = "1.40.0")]
-    #[rustc_const_unstable(feature = "const_option_match")]
-    #[cfg(not(bootstrap))]
-    pub const fn flatten(self) -> Option<T> {
-        self.and_then(convert::identity)
-    }
-
-    /// No docs for bootstrap.
-    #[inline]
-    #[stable(feature = "option_flattening", since = "1.40.0")]
-    #[cfg(bootstrap)]
     pub fn flatten(self) -> Option<T> {
         self.and_then(convert::identity)
     }

src/test/ui/consts/const-option.rs

@@ -51,8 +51,4 @@ assert_same_const! {
     const TRANSPOSE_A: Result<Option<i32>, bool> = Some(Ok(2)).transpose();
     const TRANSPOSE_B: Result<Option<i32>, bool> = Some(Err(false)).transpose();
     const TRANSPOSE_C: Result<Option<i32>, bool> = None.transpose();
-
-    const FLATTEN_A: Option<i32> = Some(Some(2)).flatten();
-    const FLATTEN_B: Option<i32> = Some(None).flatten();
-    const FLATTEN_C: Option<i32> = None.flatten();
 }