Remove legacy Travis-CI configuration.

Travis-CI is now strictly a paid service. Multiple attempts to use their "free" service have failed due to lack of community credit and general issues with their plugin.

Remove the configuration so it does not appear we are testing on Travis-CI.

.travis.yml

@@ -1,36 +0,0 @@
-branches:
-  only:
-    - integration
-    - /-ci$/
-    - /-cit$/
-
-os: linux
-dist: bionic
-
-language: c
-
-jobs:
-  # Run unit tests that provide wide coverage on multiple architectures.
-  include:
-    # Valgrind is disabled due to some platform-specific issues in getpwuid() and getgrgid() that do not seem to be pgBackRest bugs.
-    - arch: ppc64le
-      env:
-        - PGB_CI="test --vm=none --param=no-coverage --param=module=command --param=module=storage --param=no-valgrind"
-      services:
-
-    - arch: arm64
-      env:
-        - PGB_CI="test --vm=none --param=no-coverage --param=module=command --param=module=storage"
-      services:
-
-    - arch: s390x
-      env:
-        - PGB_CI="test --vm=none --param=no-coverage --param=module=command --param=module=storage"
-      services:
-
-install:
-  - umask 0022 && cd ~ && pwd && whoami && umask && groups
-  - df -Th && top -bn1
-
-script:
-  - ${TRAVIS_BUILD_DIR?}/test/ci.pl ${PGB_CI?}