LibWeb: Remove the initialize_strings methods

We added these methods to propagate OOM errors at process startup, but
we longer fret about these tiny OOM failures. Requiring that these init
methods be called prohibits using these strings in processes that have
not set up a MainThreadVM. So let's just remove them and initialize the
strings in a sane manner.

In doing so, this also standardizes how we initialize strings whose C++
variable name differs from their string value. Instead of special-casing
these strings, we just include their string value in the x-macro list.

Libraries/LibWeb/HTML/CustomElements/CustomElementReactionNames.h

@@ -25,6 +25,4 @@ namespace Web::HTML::CustomElementReactionNames {
 ENUMERATE_CUSTOM_ELEMENT_REACTION_NAMES
 #undef __ENUMERATE_CUSTOM_ELEMENT_REACTION_NAME
 
-void initialize_strings();
-
 }