Remove DeferredGarbageCollection class
Remove DeferredGarbageCollection class

This was applied 7 years ago to speed up RSpec tests. Now in 2020 I
tried it out for it had a negligible difference (3.32s with this class
for the whole test suite, 3.48s without it). Since this application
receives very little attention it doesn't seem worth keeping something
this complex.

alphagov/bouncer,alphagov/bouncer,alphagov/bouncer