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/run-docker-build.sh

@@ -47,15 +47,6 @@ else
 	else
 		useradd -u $HOST_UID $CI_USER
 	fi
-
-	# Due to a bug the test suite was run as root in the past, so
-	# a prove state file created back then is only accessible by
-	# root.  Now that bug is fixed, the test suite is run as a
-	# regular user, but the prove state file coming from Travis
-	# CI's cache might still be owned by root.
-	# Make sure that this user has rights to any cached files,
-	# including an existing prove state file.
-	test -n "$cache_dir" && chown -R $HOST_UID:$HOST_UID "$cache_dir"
 fi
 
 # Build and test