Remove some dead code.

Remove some dead code.

As the assertion at the top of the patch shows, 'starting_indices' must only have values
between 0 and N (=sparsity.n_rows()). Consequently, the loop that replaces out-of-bounds
values with numbers::invalid_size_type should never trigger. Further, the 'remove_if'
will also never do anything, and the fact that we accidentally not actually do anything
with its return value (as one should for 'remove_if') doesn't matter.