rust: syn: remove `unicode-ident` dependency

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>