da.cc: only keep track of a single set of memory based dependences

da.cc: only keep track of a single set of memory based dependences

Since a7965ed (find_sources: keep track of original converted access
relation, Mon May 7 13:21:10 2012 +0200), we had to keep track
of two sets of memory dependences, one for use within need_parametrization
and compute_sink_restriction and one for use within add_dep.
There is a difference between the two uses in find_sources as the
first work on the "actual" dependences, while the second work on
"extended" dependences with extra dimensions in the domains.

Now that we have removed all uses of memory based dependences from add_dep,
we only need to keep track of a single set of memory based dependences.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>