Rename tiered work units for removing objects

There are work units for tiered storage for removing objects either in the local database or in the shared bucket. Those workunits are named WT_TIERED_WORK_DROP_LOCAL and WT_TIERED_WORK_DROP_SHARED.

These work units are NOT used during a session->drop() call. They are perhaps poorly named and confusing. We should consider renaming the work unit macro to avoid the word DROP. Some possible naming replacements for DROP:

DELETE

DISCARD

PURGE

REMOVE

Currently the WT_TIERED_WORK_DROP_SHARED is not used. It is anticipated to be used for garbage collection. (It does have a use/line in the code to call with it to avoid script "unused" errors however.)

The WT_TIERED_WORK_DROP_LOCAL is used to remove local objects from the database when the local_retention period expires.