feat: update ai-token-ratelimit documentation by removing ai-statistics plugin

[cr7258]

Ⅰ. Describe what this PR did

Currently, ai-token-ratelimit does not rely on ai-statistics plugin, remove ai-statistics from the documentation.

Ⅱ. Does this pull request fix one issue?

fixes: https://github.com/alibaba/higress/issues/1760

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews