[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_yes/views/snippets/s_media_list.xml

@@ -42,7 +42,7 @@
         <div class="col-lg-9 s_media_list_body">
             <h3 class="card-title"><span style="font-size: 36px;">Wedding Receptions</span></h3>
             <p class="card-text">As a full service experience, we monitor all aspects of your wedding from planning to execution.</p>
-            <a href="#" class="btn btn-primary mb-2">Start planning your wedding</a>
+            <a href="#" class="btn btn-primary">Start planning your wedding</a>
         </div>
     </xpath>
     <!-- Row #2 - Image -->
@@ -61,7 +61,7 @@
         <div class="col-lg-9 s_media_list_body">
             <h3 class="card-title"><span style="font-size: 36px;">Vow renewals</span></h3>
             <p class="card-text">The heartfelt exchange of wedding vows is often the most memorable part of a marriage ceremony.</p>
-            <a href="#" class="btn btn-outline-primary mb-2">See how others renewed their vows</a>
+            <a href="#" class="btn btn-outline-primary">See how others renewed their vows</a>
         </div>
     </xpath>
     <!-- Row #3 - Image -->