[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_monglia/views/customizations.xml

@@ -119,7 +119,7 @@
     </xpath>
     <xpath expr="//p[2]" position="replace"/>
     <xpath expr="//a" position="replace">
-        <a href="/contactus" class="btn btn-primary mb-2">Learn more</a>
+        <a href="/contactus" class="btn btn-primary">Learn more</a>
     </xpath>
 </template>
 
@@ -156,7 +156,7 @@
         <p class="lead">This is the first full-length studio release in seven years for the British singer-songwriter.</p>
     </xpath>
     <xpath expr="//a" position="replace">
-        <a href="/contactus" class="btn btn-primary mb-2">Discover</a>
+        <a href="/contactus" class="btn btn-primary">Discover</a>
     </xpath>
 </template>