[FIX] theme_*: remove mb-2 class on orphan buttons

*: anelusia, artists, aviato, graphene, kiddo, loftspace, monglia,
notes, odoo_experts, paptic, test_custo, yes

The community PR [125824] introduces an automatic check on buttons to
add the `mb-2` class when several buttons follow each other, so that
they have space to breathe when split on multiple lines.
For consistency, this commit therefore removes most `mb-2` occurrences
on buttons within themes as long as they are alone on their line, but
keeps or adds the class when several buttons are next to each other.

[125824]: https://github.com/odoo/odoo/pull/125824

task-3369604

closes odoo/design-themes#680

Related: odoo/odoo#125824
Signed-off-by: Benjamin Vray (bvr) <bvr@odoo.com>

theme_test_custo/data/pages.xml

@@ -14,7 +14,7 @@
                             <h2>A Section Subtitle</h2>
                             <p>Write one or two paragraphs describing your product or services. To be successful your content needs to be useful to your readers.</p>
                             <p>Start with the customer - find out what they want and give it to them.</p>
-                            <p><a href="#" class="btn btn-primary mb-2">Learn more</a></p>
+                            <p><a href="#" class="btn btn-primary">Learn more</a></p>
                         </div>
                         <div class="col-lg-6 pt16 pb16">
                             <img src="/web_editor/image_shape/website.s_text_image_default_image/theme_test_custo/blob/01.svg" class="img img-fluid mx-auto" alt="" data-original-mimetype="image/jpeg" data-file-name="01.svg" data-shape="theme_test_custo/blob/01" data-shape-colors=";;;;"/>