Makefile: drop extra dependencies for test helpers

A few test-helpers have Makefile dependencies on specific
object files. But since these files are part of libgit.a
(which all of the helpers link against), the inclusion is
simply redundant.

These were once necessary, but became redundant due to
5c5ba73 (Makefile: Use generic rule to build test programs,
2007-05-31), which added the $(GITLIBS) dependency (but
didn't prune the extra dependency lines). Later commits then
cargo-culted the practice (e.g., b4285c7).

Note that we _do_ need to leave the dependencies on the svn
library, as that is not part of the usual link command.

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