rust: proc-macro2: remove unicode_ident dependency

The `proc-macro2` 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/proc-macro2/fallback.rs

@@ -826,11 +826,11 @@ impl Ident {
 }
 
 pub(crate) fn is_ident_start(c: char) -> bool {
-    c == '_' || unicode_ident::is_xid_start(c)
+    c == '_' || c.is_ascii_alphabetic()
 }
 
 pub(crate) fn is_ident_continue(c: char) -> bool {
-    unicode_ident::is_xid_continue(c)
+    c == '_' || c.is_ascii_alphanumeric()
 }
 
 #[track_caller]