Remove version pin for dependabot-omnibus package

[b1rdex]

This is a follow up for https://github.com/wemake-services/kira-dependencies/pull/414

As I mentioned there, we use dependabot/dependabot-core latest docker image version in .gitlab-ci.yml, so I see no point in having dependabot-omnibus locked to specific version in Gemfile.

So I removed the version constraint, and also I removed the lock file – to ensure that the latest version is required on each run

[b1rdex]

I'm not a Ruby developer, so I don't know if this is a good solution at all 😁 But for me, it looks like a right decision.