BUG: remove midrule in latex output with header=False
This bug fix addresses issue #7124. If `to_latex` is called with option `header=False`, the output should not contain a `\midrule` which is used to separate the column headers from the data.

@TomAugspurger ?
 pls add a whatsnew note for 0.17.1
 Yeah this looks good (looked at this code in a while though). @loicseguin if you could rebase and add a release note, thanks.
 Done. Let me know if there's anything else I should add.
 merged via https://github.com/pydata/pandas/commit/924c419070bc6d7c4edc6779b501377a22bce58d

thanks!
