Removed env.elements in 'before-highlightall' hook (#1968)

This removes the `env.elements` property from the environment of the `before-highlightall` hook.

**Why?**

- There is no plugin that is using it.
- It is inherently incompatible with `env.selector`. Even if a plugin using `elements` wanted to respect `selector` it would not be able to because 1) there might be plugins after it (in `before-highlightall`) extending the selector and 2) the plugin has no access to the current `container`.

**Isn't this a breaking change?**

Yes, but I don't think that this will have any impact for the above reasons and because from a user perspective, it's hard to use than just iteration over your elements and calling `Prism.highlightElement` yourself.

components/prism-core.js

@@ -178,9 +178,9 @@ var _ = {
 			selector: 'code[class*="language-"], [class*="language-"] code, code[class*="lang-"], [class*="lang-"] code'
 		};
 
-		_.hooks.run("before-highlightall", env);
+		_.hooks.run('before-highlightall', env);
 
-		var elements = env.elements || container.querySelectorAll(env.selector);
+		var elements = container.querySelectorAll(env.selector);
 
 		for (var i=0, element; element = elements[i++];) {
 			_.highlightElement(element, async === true, env.callback);

prism.js

@@ -183,9 +183,9 @@ var _ = {
 			selector: 'code[class*="language-"], [class*="language-"] code, code[class*="lang-"], [class*="lang-"] code'
 		};
 
-		_.hooks.run("before-highlightall", env);
+		_.hooks.run('before-highlightall', env);
 
-		var elements = env.elements || container.querySelectorAll(env.selector);
+		var elements = container.querySelectorAll(env.selector);
 
 		for (var i=0, element; element = elements[i++];) {
 			_.highlightElement(element, async === true, env.callback);