btrfs-progs: ci: remove travis CI scripts

The free tier of travis-ci has ended on 05/2021, remove the scripts.
Nowadays zstd has made it to all distros so we don't need to build
it from sources, reiserfs support is fading out so we don't need to
build it either. The integration with coverity scan needs to be done
again. The rest has been moved to new github actions CI.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

ci/.travis.yml

@@ -55,10 +55,6 @@ were experimenting with nested virtualization to run the tests on a current
 kernel not some old version provided by the hosted image. The tests took to
 long to fit in the free plan quota.
 
-**Travis CI**
-
-Disabled since version v5.12 (05/2021).
-
 ## Local
 
 The testsuite can be run directly from the git tree from the built sources, or