.builds: remove debian.yml

io/fs cannot be imported from Go 1.15, and the default version of
Go in Debian is 1.15.

As the requried packages are almost the same as Ubuntu, tests for
Debian might be redundant. Remove it.

Updates #2323

.builds/debian.yml

@@ -1,19 +0,0 @@
-image: debian/stable
-packages:
-  - libasound2-dev
-  - libc6-dev
-  - libglu1-mesa-dev
-  - libgl1-mesa-dev
-  - libxcursor-dev
-  - libxi-dev
-  - libxinerama-dev
-  - libxrandr-dev
-  - libxxf86vm-dev
-  - pkg-config
-  - golang
-sources:
-  - https://github.com/hajimehoshi/ebiten
-tasks:
-  - build: |
-      cd ebiten
-      go build -tags=example ./...