CI: remove Travis CI support

Remove support for running the CI in travis. The last builds in it are
from 5 months ago[1] (as of 2021-11-19), and our documentation has
referred to GitHub CI instead since f003a91f5c (SubmittingPatches:
replace discussion of Travis with GitHub Actions, 2021-07-22).

We'll now run the "t9810 t9816" and tests on OSX. We didn't before, as
we'd carried the Travis exclusion of them forward from
522354d70f (Add Travis CI support, 2015-11-27). Let's hope whatever
issue there was with them was either Travis specific, or fixed since
then (I'm not sure).

The "apt-add-repository" invocation (which we were doing in GitHub CI)
isn't needed, it was another Travis-only case that was carried forward
into more general code. See 0f0c51181d (travis-ci: install packages
in 'ci/install-dependencies.sh', 2018-11-01).

Remove the "linux-gcc-4.8" job added in fb9d7431cf (travis-ci: build
with GCC 4.8 as well, 2019-07-18), it only ran in Travis CI.

1. https://travis-ci.org/github/git/git/builds

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/print-test-failures.sh

@@ -39,8 +39,6 @@ do
 		test_name="${test_name##*/}"
 		trash_dir="trash directory.$test_name"
 		case "$CI_TYPE" in
-		travis)
-			;;
 		azure-pipelines)
 			mkdir -p failed-test-artifacts
 			mv "$trash_dir" failed-test-artifacts
@@ -88,11 +86,3 @@ do
 		fi
 	fi
 done
-
-if [ $combined_trash_size -gt 0 ]
-then
-	echo "------------------------------------------------------------------------"
-	echo "Trash directories embedded in this log can be extracted by running:"
-	echo
-	echo "  curl https://api.travis-ci.org/v3/job/$TRAVIS_JOB_ID/log.txt |./ci/util/extract-trash-dirs.sh"
-fi