[mod] simple-theme: remove vendor CSS

flexbox
  Browser support for flexbox is excellent, and the majority of browsers do not
  need a prefix at this point. Safari was the last of the major browsers to
  remove prefixes, with the release of Safari 9 in 2015. [1]

user-select:
  Vendor prefix of user-select is not needed, see 'Browser compatibility' [2].

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Backwards_Compatibility_of_Flexbox#status_in_browsers
[2] https://developer.mozilla.org/en-US/docs/Web/CSS/user-select#browser_compatibility



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>