Remove Git's support for smoke testing

Remove Git's support for smoke testing

I'm no longer running the Git smoke testing service at
smoke.git.nix.is due to Smolder being a fragile piece of software not
having time to follow through on making it easy for third parties to
run and submit their own smoke tests.

So remove the support in Git for sending smoke tests to
smoke.git.nix.is, it's still easy to modify the test suite to submit
smokes somewhere else.

This reverts the following commits:

    Revert "t/README: Add SMOKE_{COMMENT,TAGS}= to smoke_report target" -- e38efac87d
    Revert "t/README: Document the Smoke testing" -- d15e9ebc5c
    Revert "t/Makefile: Create test-results dir for smoke target" -- 617344d77b
    Revert "tests: Infrastructure for Git smoke testing" -- b6b84d1b74

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