The `modification_time` column is hard to maintain with any level of
consistency over merges and splits of invalidation ranges so this
commit removes it from the invalidation log entries for both
hypertables and continuous aggregates. If the modification time is
needed in the future, we need to re-introduce it in a manner that can
maintain it over both merges and splits.
THe function `ts_get_now_internal` is also removed since it is not used
any more.
Part of #2521
0e507affc1