Fix incorrect assert in Scatterer

[jpdean]

After discussing with @jorgensd, it looks like this assert statement might be incorrect. local_data has size size_local() * bs(), but the indices in _local_inds are grouped by the processes in the neighbourhood, so for some index maps, _local_inds.size() may be larger than local_data.size().

This PR removes the assert statement, but perhaps it could be replaced.

[IgorBaratta]

You're right, the assert statement is incorrect.
The correct statement would be something like:

assert(*std::max_element(_local_inds.begin(), _local_inds.end()) < std::int32_t(local_data.size()));