core: Remove #[macro_export] from debug_assert_matches

The `debug_assert_matches` macro was marked with the `#[macro_export]` attribute,
despite being a declarative macro/macro 2.0, for which the exporting rules are similar
to items. In fact, `#[macro_export]` on a decl macro has no effect on its visibility.

library/core/src/macros/mod.rs

@@ -312,7 +312,6 @@ macro_rules! debug_assert_ne {
 /// let c = Ok("abc".to_string());
 /// debug_assert_matches!(c, Ok(x) | Err(x) if x.len() < 100);
 /// ```
-#[macro_export]
 #[unstable(feature = "assert_matches", issue = "82775")]
 #[allow_internal_unstable(assert_matches)]
 #[rustc_macro_transparency = "semitransparent"]