Remove the "25% of active pool" growth rate throttle from Drydock

Summary:
Ref T13677. Drydock has a hard-coded and fairly arbitrary limit which prevents a resource pool from growing more than 25% at once.

This is vaguely reasonable for resources which allocate quickly, but suffocating for slower resources. It's also wholly arbitrary, and the "one per lease" limit introduced in D21807 should do a better job of covering the same ground while generally being more reasonable, expected, and predictable.

Test Plan: Ran Drydock allocations without the throttle, saw faster pool growth.

Subscribers: yelirekim, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13677

Differential Revision: https://secure.phabricator.com/D21808