update(sidenav): remove unnecessary selectors (#2460)

References #2158

Author: devversion

src/lib/sidenav/sidenav.scss

@@ -17,15 +17,10 @@
     // section.
     visibility: hidden;
   }
-  &.md-sidenav-closing {
-    transform: translate3d($close, 0, 0);
-  }
-  &.md-sidenav-opening {
-    @include md-elevation(16);
-    visibility: visible;
-    transform: translate3d($open, 0, 0);
-  }
-  &.md-sidenav-opened {
+
+  &.md-sidenav-opening, &.md-sidenav-opened {
+    // The elevation of z-16 is noted in the design specifications.
+    // See https://material.io/guidelines/patterns/navigation-drawer.html#
     @include md-elevation(16);
     transform: translate3d($open, 0, 0);
   }