jid: Remove the std feature

Since Rust 1.81, std::error::Error has been moved to core::error::Error,
which we can use without depending on std.

jid/Cargo.toml

@@ -32,6 +32,5 @@ serde_test = "1"
 jid = { path = ".", features = [ "serde" ] }
 
 [features]
-default = ["std"]
-std = []
+default = []
 quote = ["dep:quote", "dep:proc-macro2"]