ci: drop Travis CI integration

In short: deployment jobs often fail now and we'll recreate them on
GitHub Actions in a future.

In details:

* The old Travis CI infrastructure (.org) will gone soon, the new one
  (.com) has tight limits on a free plan.
* We already have testing on GitHub Actions (but without RPM / Deb
  uploads).
* Pulls from Docker Hub (part of RPM / Deb build + deploy jobs) often
  ratelimited when performed from Travis CI: likely due to reusing of IP
  addresses. GitHub Actions is not affected by this problem.
* We don't use packagecloud.io for hosting tarantool repositories
  anymore (see [1], [2], [3]). We'll deploy packages to our new infra in
  a future: it is tracked by tarantool#43.

[1]: tarantool/tarantool#3380
[2]: tarantool/tarantool#5494
[3]: tarantool/tarantool#4947

Removed unused Jenkinsfile as well.