chore: drop the unused datafusion-proto dep from the util crate

Its only use was `physical_codec_from_pycapsule`, whose `dyn
PhysicalExtensionCodec` output type came from that crate. That helper is
gone, replaced by `ffi_physical_codec_from_pycapsule`, which returns the FFI
type and leaves the conversion to the caller. Nothing in `crates/util/src`
references `datafusion_proto` any more, and it is not re-exported, so
extension libraries depending on this crate are unaffected.

The package stays in Cargo.lock; crates/core still uses it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

diff --git a/Cargo.lock b/Cargo.lock
index 72b880a..c763273 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1579,7 +1579,6 @@
  "arrow",
  "datafusion",
  "datafusion-ffi",
- "datafusion-proto",
  "prost",
  "pyo3",
  "tokio",

crates/util/Cargo.toml

diff --git a/crates/util/Cargo.toml b/crates/util/Cargo.toml
index c23667b..00d5946 100644
--- a/crates/util/Cargo.toml
+++ b/crates/util/Cargo.toml
@@ -30,6 +30,5 @@
 pyo3 = { workspace = true }
 datafusion = { workspace = true }
 datafusion-ffi = { workspace = true }
-datafusion-proto = { workspace = true }
 arrow = { workspace = true }
 prost = { workspace = true }