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 torvalds#31.

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

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/syn/ident.rs

@@ -39,11 +39,11 @@ impl From<Token![_]> for 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;
         }
     }