Remove block_size and point_source_block_size

See https://bugs.launchpad.net/oq-engine/+bug/1268070 for an explanation of why this is a good thing.

Notice that this approach does not solve the long standing problem of slow sources, it may be necessary to provide a custom openquake.cfg configuration file with a bigger parameter concurrent_tasks to produce tasks with less sources.

Tests running on http://ci.openquake.org/job/zdevel_oq-engine/253/