Remove unneeded rc feature of serde (#990)

Fixes #989.

This feature opts into impls for `Rc` and `Arc`, but none of the data
structures that use Serialize/Deserialize actually contain `Rc` or
`Arc`s.

See:

- [Serde docs](https://serde.rs/feature-flags.html#-features-rc)
- [PR adding this](apache/arrow#3016)

arrow/Cargo.toml

@@ -36,7 +36,7 @@ name = "arrow"
 path = "src/lib.rs"
 
 [dependencies]
-serde = { version = "1.0", features = ["rc"] }
+serde = { version = "1.0" }
 serde_derive = "1.0"
 serde_json = { version = "1.0", features = ["preserve_order"] }
 indexmap = "1.6"