rust: syn: remove unicode-ident dependency

The `syn` crate depends on the `unicode-ident` crate to determine whether
characters have the XID_Start or XID_Continue properties according to
Unicode Standard Annex #31.

However, we only need ASCII identifiers in the kernel, thus we can
simplify the check and remove completely that dependency.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Gary Guo <gary@garyguo.net>
Tested-by: Jesung Yang <y.j3ms.n@gmail.com>
Link: https://patch.msgid.link/20251124151837.2184382-18-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/syn/ident.rs

@@ -39,11 +39,11 @@ fn from(token: Token![_]) -> Ident {
 pub(crate) fn xid_ok(symbol: &str) -> bool {
     let mut chars = symbol.chars();
     let first = chars.next().unwrap();
-    if !(first == '_' || unicode_ident::is_xid_start(first)) {
+    if !(first == '_' || first.is_ascii_alphabetic()) {
         return false;
     }
     for ch in chars {
-        if !unicode_ident::is_xid_continue(ch) {
+        if !(ch == '_' || ch.is_ascii_alphanumeric()) {
             return false;
         }
     }