t/Makefile: remove smoke test targets

Commit d24fbca (Remove Git's support for smoke testing - 2011-12-23)
removed the smoke test support from the test suite but it was
re-added by commit 342e9ef (Introduce a performance testing
framework - 2012-02-17).  This appears to be the result of a
mis-rebase, since re-adding the smoke testing infrastructure does
not relate to the subject of that commit.

The current 'smoke' target is broken since the 'harness' script it
uses no longer exists, so just reapply this section of commit d24fbca
and remove all of the smoke testing section in the makefile.

Signed-off-by: John Keeping <john@keeping.me.uk>
Acked-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
documentation: trivial whitespace cleanups

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>