isl_tab_pip.c: add_lexmin_eq: remove misguided optimizations

isl_tab_pip.c: add_lexmin_eq: remove misguided optimizations

After adding a pair of inequalities corresponding to the equality,
the code tries to reduce the dimensionality of the problem by
removing a column.  If one of the added constraints happens to have
been pivoted into a column, then it can easily be removed.
Otherwise, the code tries to pivot the row into a column and
then remove the column.

This pivot may disrupt the lexico-positiveness of the columns however.
At the first least, the code should check that the pivot entry
is negative, but it should also avoid pivoting out a column corresponding
to a parameter.

Since this optimization shouldn't have that much of an effect anyway,
we choose here to just remove it for now.

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