isl_basic_set_sample: don't project out lineality space up front

isl_basic_set_sample: don't project out lineality space up front

For the pip based sampler, projecting out the lineality space
is part of isl_basic_set_skew_to_positive_orthant, so there is
no need for a separate projection step before the skewing.

For the gbr based sampler, there is no need to project out the lineality
space.  The lineality space will simply be part of the recession cone,
which is handled more efficiently than the projection.
In particular, for systems with many variables, the unimodular transformation
used in the projection may have very large coefficients, which in turn
leads to constraints with large coefficients.  Since there is no need
to do this projection, it is best to avoid it.