Remove unnecessary and bug-triggering style rules #cleanup

These (actually, just the width) are triggering a bug in Webkit causing
the whole page to jump around:
https://github.com/mathquill/mathquill/pull/261#issuecomment-38333601

And are totally unnecessary, the textarea is outside of and can't affect
the scrollWidth of the .mathquill-root-block:
https://github.com/mathquill/mathquill/pull/261#commitcomment-5763880