ci: remove SDL2 bench build as we have removed it long ago

.woodpecker/build_sdl2.yaml

@@ -1,16 +0,0 @@
-matrix:
-  include:
-    - CHANNEL: stable
-      FEATURES: sdl2,disable_contracts
-    - CHANNEL: nightly
-      FEATURES: sdl2
-
-steps:
-  build_sdl2:
-    image: rust
-    environment: [CARGO_TERM_COLOR=always]
-    commands:
-      - apt-get update
-      - apt-get -y install cmake
-      - rustup default "$CHANNEL"
-      - cargo build --features "$FEATURES" --bin "topola-sdl2-demo"