Remove unused rmcp client dependency

Drop the stale codex-client dependency from codex-rmcp-client after the OAuth transport changes stopped using that crate.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/Cargo.lock

@@ -3168,7 +3168,6 @@ dependencies = [
  "axum",
  "bytes",
  "codex-api",
- "codex-client",
  "codex-config",
  "codex-exec-server",
  "codex-keyring-store",

codex-rs/rmcp-client/Cargo.toml

@@ -14,7 +14,6 @@ axum = { workspace = true, default-features = false, features = [
     "tokio",
 ] }
 codex-api = { workspace = true }
-codex-client = { workspace = true }
 codex-config = { workspace = true }
 codex-exec-server = { workspace = true }
 codex-keyring-store = { workspace = true }