pw_containers: Remove redundant DequeIterator checks

- Dereferencing a deque iterator calls container_->at(), which performs
  an index check, so it's not necessary to check the index before
  calling at().
- Switch to a reference constructor since its simpler and the container
  should never be nullptr unless the iterator was default constructed.

Change-Id: I0ae8c71f4be94d0661b18a4ba858ef5d88618977
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/301692
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
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Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Reviewed-by: Dave Roth <davidroth@google.com>
Docs-Not-Needed: Wyatt Hepler <hepler@google.com>
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Commit-Queue: Wyatt Hepler <hepler@google.com>