search: Simplify mjolnir dag

This was the first airflow dag built for the search platform, and it's also our most complex. As such it experimented with a few different ideas, but based on what we've learned about using airflow since then it is far more complex than necessary.

Simplify away much of the complexity in the mjolnir tag. Remove the MjolnirTask object. Stop trying to pass partitioning information from task to task. Instead declare where all the data should go up front, and reuse those declarations as the outputs and the inputs where appropriate. Similarly, stop breaking up the partitioning information across multiple cli arguments. Instead use the HivePartition spec's we use everywhere else to specify where data should be read or written.

Bug: T390061