ARROW-8396: [Rust] Removes libc dependency

Since the allocation has done by std::alloc::*;

Closes #6896 from vertexclique/ARROW-8396-remove-libc-dep

Authored-by: Mahmut Bulut <vertexclique@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>

rust/arrow/Cargo.toml

@@ -36,7 +36,6 @@ name = "arrow"
 path = "src/lib.rs"
 
 [dependencies]
-libc = "0.2"
 serde = { version = "1.0", features = ["rc"] }
 serde_derive = "1.0"
 serde_json = { version = "1.0", features = ["preserve_order"] }