Remove travis config file (#7088)

Due to changes in the travis terms of service and the very limited
quotas they offer to open source projects [1] now we haven't been running
travis jobs for months due to exceeding the quota. Additionally due to
the recent security issues [2] we've disabled travis as a service on the
qiskit organization as it wasn't being used anymore and exposed a risk.
Since we're not using travis anymore and won't be in the future this
commit just deletes the travis config file to avoid confusion.

[1] https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
[2] https://nvd.nist.gov/vuln/detail/CVE-2021-41077

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>