ci: drop linux-clang job

ci: drop linux-clang job

Since the linux-asan-ubsan job runs using clang under Linux, there is
not much point in running a separate clang job. Any errors that a normal
clang compile-and-test cycle would find are likely to be a subset of
what the sanitizer job will find. Since this job takes ~14 minutes to
run in CI, this shaves off some of our CPU load (though it does not
affect end-to-end runtime, since it's typically run in parallel and is
not the longest job).

Technically this provides us with slightly less signal for a given run,
since you won't immediately know if a failure in the sanitizer job is
from using clang or from the sanitizers themselves. But it's generally
obvious from the logs, and anyway your next step would be to fix the
probvlem and re-run CI, since we expect all of these jobs to pass
normally.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>