Strip some obsolete code out of Drydock

Summary:
Ref T9252. This simplifies some Drydock code.

Most of this code relates to the old notion of Drydock being able to enumerate all the tasks it needs to complete in order to acquire a lease. The code has stepped back from this, since it's unnecessary, the queue is more powerful than it used to be, and it would be a lot of work to keep track of.

The ~only thing that should ever wait for leases in modern code is bin/drydock lease, and it's fine for it to just sit there sleeping, so this just does that.

This reduces the granularity of logging, but I'll address that separately in future logging-focused changes.

Test Plan: Used bin/drydock lease to acquire a lease, saw it acquire cleanly.

Reviewers: hach-que, chad

Reviewed By: chad

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14147