ci: Remove empty workflow file

In commit e45f7484f2de ("ci: Split job, avoid running
conformance/coverage w/ all toolchains") we split the job in the CI
workflow. One initial version of that change consisted in splitting into
two distinct files. Eventually, both jobs were kept in the same file,
but it seems we mistakenly committed an empty file in the process. Let's
remove it.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>