[feature/prosilver-cleanup/remove-tweaks2] Tweaks.css only for IE

Removing tweaks.css from stylesheet.css, including it for IE8 or older via conditional statements

PHPBB3-10622

phpBB/styles/prosilver/theme/stylesheet.css

@@ -13,6 +13,5 @@
 @import url("buttons.css");
 @import url("cp.css");
 @import url("forms.css");
-@import url("tweaks.css");
 @import url("colours.css");
 @import url("imageset.css");