Remove LOG_PREFIX variable

This was used in times when the same code was executed from different places. We don't do that anymore, so the variable is obsolete.

build-pipeline-ci-test.yml

@@ -39,7 +39,6 @@ build:files:
   timeout: 2m
   variables:
     GIT_STRATEGY: none
-    LOG_PREFIX: "CI:build:"
   before_script:
     - !reference [.docker_check]
     - !reference [.setup_ssh]