db: schema_tables: futurize and coroutinize user_types_to_drop::drop
db: schema_tables: futurize and coroutinize user_types_to_drop::drop

user_types_to_drop::drop is a function object returning void, and expecting
to be called in a thread. Make it return a future and convert the
only value it is initialized to to a coroutine.

De-threading helps reduce errors where something expects to be called
in a thread, but isn't.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla