remove ember-destroyable-polyfill dependency #581

Job	Run time
Docs	1m 22s
Tests - Basic App	4m 41s
Tests	1m 7s
Floating Dependencies	1m 20s
Tests - App That Excludes Mirage	1m 6s
ember-lts-3.28	1m 5s
ember-lts-4.4	1m 15s
ember-lts-4.8	1m 6s
ember-lts-4.12	1m 11s
ember-release	1m 9s
ember-beta	1m 15s
ember-canary	1m 3s
	17m 40s

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