Remove transform-style workaround in eab984de

Signed-off-by: Yukai Huang <yukaihuangtw@gmail.com>

public/css/slide.css

@@ -258,19 +258,10 @@ pre.vega > svg {
     margin-bottom: -.25em !important;
 }
 
-.reveal .slides, .reveal .backgrounds, .reveal.overview {
-    transform-style: preserve-3d;
-}
-
 .slides, #meta {
     display: none;
 }
 
-.reveal .slides > section,
-.reveal .slides > section > section {
-    transform-style: flat;
-}
-
 .reveal.rtl .slides,
 .reveal.rtl .slides h1,
 .reveal.rtl .slides h2,