remove another ci cache

.drone.yml

@@ -9,23 +9,14 @@ concurrency:
 steps:
 - name: build
   image: rustlang/rust:nightly
-  volumes:
-  - name: cache
-    path: /usr/local/cargo
   commands:
   - cargo build
 - name: clippy!
   image: rustlang/rust:nightly
-  volumes:
-  - name: cache
-    path: /usr/local/cargo
   commands:
   - cargo clippy -- --deny warnings
 - name: test
   image: rustlang/rust:nightly
-  volumes:
-  - name: cache
-    path: /usr/local/cargo
   commands:
   - cargo test --jobs 1