SDK / exoplayer

The assertion was so weak it probably wouldn't detect genuine
misuse of the DefaultAllocator API, so it seems fine just to
remove it.

We don't really know what happens when the player is allowed to
continue on the affected devices, but hopefully it either "just
works" or fails in a more graceful way.

Issue: #4532

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217113060