chore: Remove support for concat_idents!

`concat_idents!` was deprecated in [1] and will be removed in the near
future. rust-analyzer's support is independent of rustc's, so drop RA
support now to make syncing easier.

[1]: https://github.com/rust-lang/rust/pull/137653

crates/hir-def/src/macro_expansion_tests/builtin_fn_macro.rs

@@ -509,24 +509,6 @@ fn main() { "s"; }
     );
 }
 
-#[test]
-fn test_concat_idents_expand() {
-    check(
-        r##"
-#[rustc_builtin_macro]
-macro_rules! concat_idents {}
-
-fn main() { concat_idents!(foo, bar); }
-"##,
-        expect![[r##"
-#[rustc_builtin_macro]
-macro_rules! concat_idents {}
-
-fn main() { foobar; }
-"##]],
-    );
-}
-
 #[test]
 fn test_quote_string() {
     check(

crates/hir-expand/src/builtin/fn_macro.rs

@@ -140,7 +140,6 @@ register_builtin! {
     EagerExpander:
     (compile_error, CompileError) => compile_error_expand,
     (concat, Concat) => concat_expand,
-    (concat_idents, ConcatIdents) => concat_idents_expand,
     (concat_bytes, ConcatBytes) => concat_bytes_expand,
     (include, Include) => include_expand,
     (include_bytes, IncludeBytes) => include_bytes_expand,
@@ -660,30 +659,6 @@ fn concat_bytes_expand_subtree(
     Ok(())
 }
 
-fn concat_idents_expand(
-    _db: &dyn ExpandDatabase,
-    _arg_id: MacroCallId,
-    tt: &tt::TopSubtree,
-    span: Span,
-) -> ExpandResult<tt::TopSubtree> {
-    let mut err = None;
-    let mut ident = String::new();
-    for (i, t) in tt.iter().enumerate() {
-        match t {
-            TtElement::Leaf(tt::Leaf::Ident(id)) => {
-                ident.push_str(id.sym.as_str());
-            }
-            TtElement::Leaf(tt::Leaf::Punct(punct)) if i % 2 == 1 && punct.char == ',' => (),
-            _ => {
-                err.get_or_insert(ExpandError::other(span, "unexpected token"));
-            }
-        }
-    }
-    // FIXME merge spans
-    let ident = tt::Ident { sym: Symbol::intern(&ident), span, is_raw: tt::IdentIsRaw::No };
-    ExpandResult { value: quote!(span =>#ident), err }
-}
-
 fn relative_file(
     db: &dyn ExpandDatabase,
     call_id: MacroCallId,

crates/intern/src/symbol/symbols.rs

@@ -164,7 +164,6 @@ define_symbols! {
     completion,
     compile_error,
     concat_bytes,
-    concat_idents,
     concat,
     const_format_args,
     const_panic_fmt,