[Feature request] Remove header field in page data

[Mister-Hope]

Clear and concise description of the problem

Now the downstream is no longer using header in page data (note: headers should still be on the page object).

Suggested solution

Since @vuepress/helper provides a way to get page headers from page contents, we 'd better remove it from page data to reduce client side data size.

Alternative

No response

Additional context

No response