Use user set rate when reading math.

Use user set rate when reading math.

As part of feedback received from users, no longer use absolutespeech rate when reading math. The reasoning for this functionality mainly centered around using rate as a "dimension" to convey expression/subexpressions. However, it turns out we don't use rate often in the math speech rules.

TEST=verify speech generated for math respects user speech rate.

Review URL: https://codereview.chromium.org/852773003

Cr-Commit-Position: refs/heads/master@{#311696}