Remove MetadataBulkLoadFilter

[keith-turner]

This is follow on work to #3350. The MetadataBulkLoadFilter was created to handle the case where tablets would split concurrently with a bulk load fate operation resulting in race conditions where load flags being left in the metadata table after the FATE operation had completed. The changes in #3350 modify the bulk load fate operation to use conditional mutations. This change should allow concurrent splits to be detected and handled by the bulk load fate operation. That should remove the need for the MetadataBulkLoadFilter. We could wait until after the split code is moved from the tserver to the manager to have more certainty that this code is no longer needed.

Removing the MetadataBulkLoadFilter will involve a bit more than just removing the code. This filter is configured on the metadata table so Accumulo's upgrade and initialization code will need to be updated to adjust the metadata table config.