Remove 'only' keyword statements

It's unclear (to me) what they filter out. And why.

https://salsa.debian.org/help/ci/yaml/index#only-except says:
"NOTE: only and except are not being actively developed. rules is
the preferred keyword to control when to add jobs to pipelines."

So if filtering is desired, it should be added using the 'rules' keyword
and the reasoning behind it should be clarified in comments and/or
commit messages.

debian/salsa-ci.yml

@@ -10,10 +10,6 @@ variables:
 build-yamls:
   stage: build
   image: $BASE_CI_IMAGES:unstable
-  only:
-    - branches
-    - master # I suppose that we can work directly on master
-    - merge_request
   dependencies: []
   script:
     - apt-get update && apt-get install -y vmdb2 dosfstools qemu-utils qemu-user-static debootstrap binfmt-support time kpartx bmap-tools python3 fakemachine make