gitlab-ci: remove tag from pushed branch commit

Drop a tag that points to a current commit (if any) on a job triggered
by pushing to a branch (as against of pushing a tag). Otherwise we may
get two jobs for the same x.y.z-0-gxxxxxxxxx build: one is run by
pushing a branch and another by pushing a tag. The idea is to hide the
new tag from the branch job as if a tag would be pushed strictly after
all branch jobs for the same commit.

Closes #3745

Co-authored-by: Alexander Turenko <alexander.turenko@tarantool.org>