Don't replace the stylesheet when option adaptiveCSS is false

Created by: mt4c

Issue Summary

When the adaptiveCSS is false, there is no need to replace the stylesheet ( the "MJX-CHTML-styles" element ) on typesetting. It cause flicker on old version browsers and slow devices.

Steps to Reproduce:

1. Typeset some math on the page.
2. Typeset some other math on the page. The old maths flicker.

Technical details:

• MathJax Version: 3
• Client OS: Android
• Browser: Chrome 52

Supporting information:

I copy the stylesheet and modify the font-family name. Make the font family styles "!important". Insert it in the page. Then the flickers disappear.