ci: remove travis CI

the Travis CI has worked for years but eventually required to get credits for FOSS projects
to execute. We've tried to add credits and it worked several times but always required manual
exchange of email between us and support staff.

Since github actions now include the same and even more checks without any extra work
we are going to remove Travis CI from srsRAN.

.travis.yml

@@ -1,31 +0,0 @@
-dist: bionic
-sudo: required
-
-before_script:
-   - sudo apt-get -qq update
-   - sudo apt-get install -qq build-essential cmake libfftw3-dev libmbedtls-dev libpcsclite-dev libboost-program-options-dev libconfig++-dev libsctp-dev libczmq-dev libpcsclite-dev rapidjson-dev colordiff ninja-build clang-format-8
-
-language: cpp
-        
-compiler:
-  - gcc
-  - clang
-
-script:
- - sudo ln -s /usr/bin/clang-format-diff-8 /usr/bin/clang-format-diff
- - git remote set-branches --add origin master
- - git fetch
- - |
-   if [[ "$TRAVIS_PULL_REQUEST" != "false" ]]; then
-     # Run only for PRs because target branch is needed to do the clang-format check
-     echo "Checking clang-format between TRAVIS_BRANCH=$TRAVIS_BRANCH and TRAVIS_PULL_REQUEST_BRANCH=$TRAVIS_PULL_REQUEST_BRANCH"
-     ./run-clang-format-diff.sh "$TRAVIS_BRANCH" "$TRAVIS_PULL_REQUEST_BRANCH"
-   else
-     echo "Skipping clang-format check"
-   fi
- - mkdir build
- - cd build
- - cmake -DENABLE_TTCN3=True -DRF_FOUND=True -G Ninja ..
- - ninja
- - ninja test
- - sudo ninja install