CSS: Remove the opacity CSS hook

The consequence is `.css( "opacity" )` will now return an empty string for
detached elements in standard-compliant browsers and "1" in IE & the legacy
Edge. That behavior is shared by most other CSS properties which we're not
normalizing either.

Closes gh-4593

Author: mgol

src/css.js

@@ -185,18 +185,7 @@ jQuery.extend( {
 
 	// Add in style property hooks for overriding the default
 	// behavior of getting and setting a style property
-	cssHooks: {
-		opacity: {
-			get: function( elem, computed ) {
-				if ( computed ) {
-
-					// We should always get a number back from opacity
-					var ret = curCSS( elem, "opacity" );
-					return ret === "" ? "1" : ret;
-				}
-			}
-		}
-	},
+	cssHooks: {},
 
 	// Get and set the style property on a DOM Node
 	style: function( elem, name, value, extra ) {