Remove once_cell dependency (#31493)

removing once_cell dep imported from #31439

it should work just fine

Release Notes:

- N/A

crates/assistant_tools/Cargo.toml

@@ -62,7 +62,6 @@ which.workspace = true
 workspace-hack.workspace = true
 workspace.workspace = true
 zed_llm_client.workspace = true
-once_cell = "1.21.3"
 
 [dev-dependencies]
 client = { workspace = true, features = ["test-support"] }