README: drop Travis CI badge

We stopped using Travis as of:
commit c66ccb6
    ci: Remove Travis CI

Signed-off-by: Ján Tomko <[email protected]>

Author: janotomko

README.rst

@@ -1,9 +1,6 @@
 .. image:: https://gitlab.com/libvirt/libvirt/badges/master/pipeline.svg
      :target: https://gitlab.com/libvirt/libvirt/pipelines
      :alt: GitLab CI Build Status
-.. image:: https://travis-ci.org/libvirt/libvirt.svg
-     :target: https://travis-ci.org/libvirt/libvirt
-     :alt: Travis CI Build Status
 .. image:: https://bestpractices.coreinfrastructure.org/projects/355/badge
      :target: https://bestpractices.coreinfrastructure.org/projects/355
      :alt: CII Best Practices