`AdaptiveModeLevelEstimator::last_level_dbfs_` doesn't need to be optional. Note: this CL breaks the chain of 3 CLs titled "AGC2 AdaptiveModeLevelEstimator min consecutive speech frames". Bug: webrtc:7494 Change-Id: Id5b409ca5cb5f11ed132c861b7995b9721e167bb Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185809 Reviewed-by: Minyue Li <minyue@webrtc.org> Commit-Queue: Alessio Bazzica <alessiob@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32237}