Revert "WorkOrderSearchForm: add desc indexes to complement asc ones"

This reverts commit e9f1b27711540d78c3d2bf8a557cdff7a1554a0f.
Revert "WorkOrderSearchForm: add more indexes"

This reverts commit 8e00e9680f5970ba8825442a067458ebf5e25f20.
Revert "New admin: update indexes for work order searches"

This reverts commit 013ae9d95690af5c7d202e7a9a33c6c5f97e8385.
Manually specify indexes on Site

For WorkOrderSearchForm and similar to use 'zigzag merging' and
avoid combinatorial definition of indexes

Structure: for all filterable fields f, for all sort orders o,
create index for (f, o ascending), (f, o descending)

For rationale, see https://developers.google.com/appengine/articles/indexselection