Refactor book module CSS files inline with our CSS standards

@crazyrohila Thanks for the patch, we actually have different file naming standards with modules than we do with themes, see the coding standards docs. I think we are better off grouping them under book.theme.css for now. It also looks possible to reduce the component to just one, we could probably clean up the template a little and remove unused classes.

1. +++ b/core/modules/book/css/components/navigation.css
   @@ -0,0 +1,9 @@
   +.book-navigation .menu {
   

   Is this styling used? I don't see a menu class but I do see the book-pager class, maybe we can just use that?

2. +++ b/core/modules/book/css/components/pager.css
   @@ -0,0 +1,36 @@
   +.book-navigation .book-pager {
   

   Maybe we can remove the book-navigation class and just have book pager?

.js .book-outline-form .form-submit {
  display: none;
}

I think this might be a good opportunity to introduce a reuseable class to achieve this called js-hidden.

Nice! I tested the patch to make sure the styling still applies and it does. We can tidy up the old classes though, I noticed that they were still printing now they aren't being used.

+++ b/core/modules/book/book.module
@@ -163,6 +163,11 @@ function book_form_node_form_alter(&$form, FormStateInterface $form_state, $form
diff --git a/core/modules/book/css/book.admin.css b/core/modules/book/css/book.admin.css

diff --git a/core/modules/book/css/book.admin.css b/core/modules/book/css/book.admin.css
deleted file mode 100644

We've deleted this file but we also need to remove reference to this file so it's not being loaded, we can remove the admin library in book.libraries.yml and also remove reference to that library:

'#attached' => [
        'library' => [
          'book/admin',
        ],
      ],

    // Since the "Book" dropdown can't trigger a form submission when
    // JavaScript is disabled, add a submit button to do that. book.admin.css hides
    // this button when JavaScript is enabled.

We need to update this comment to: " the js-hide class hides this button when Javascript is enabled"

I thought they can be useful as fallback as if I am not aware of all places where they have been used. Removed Now.

We can still remove classes before we hit release candidate 1.

+++ b/core/modules/book/book.module
@@ -150,19 +150,12 @@ function book_form_node_form_alter(&$form, FormStateInterface $form_state, $form
+    // The "js-hide" class hides submit button when Javascript is enabled

Ah almost! We need a full stop at the end of this comment :( Then we are done!

Great, thanks!