primal_heur.cpp: do not check global bounds

Global bounds might be too strict, since they are inferred
from a subset of variables (columns). In other words, those bounds
have little meaning for the original problem.
Therefore, solutions obtained by primal heuristics need not
respect those bounds.

Similarly, one could think that propagating the global bounds in
the restricted MILP could be of some help. However, for the very
same reason explained above, one would rather *not* include those
bounds. (Although, in principle, the solution found by the current
RMILP should not violate those bounds.

Also: fix subtle bug in restricted MILP. Looping over the paths
can be incorrect when the RMILP runs after the shortest path
heuristic, since the number of paths might be larger than the
number of lambda variables actually in the problem.
Always loop on lambdas, then.