[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_odoo_experts/views/snippets/s_picture.xml

@@ -30,7 +30,7 @@
     <!-- Add a button -->
     <xpath expr="//p[2]" position="after">
         <p style="text-align: center;">
-            <a t-att-href="cta_btn_href" class="btn btn-primary mb-2 rounded-circle btn-lg"><t t-esc="cta_btn_text">Contact Us</t></a>
+            <a t-att-href="cta_btn_href" class="btn btn-primary rounded-circle btn-lg"><t t-esc="cta_btn_text">Contact Us</t></a>
         </p>
     </xpath>
     <!-- Image -->