[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_paptic/views/customizations.xml

@@ -105,7 +105,7 @@
     </xpath>
 
     <xpath expr="//a[hasclass('btn')]" position="replace">
-        <a href="#" class="btn btn-link mb-2">DISCOVER MORE &#160;<i class="fa fa-chevron-right"/></a>
+        <a href="#" class="btn btn-link">DISCOVER MORE &#160;<i class="fa fa-chevron-right"/></a>
     </xpath>
 
     <xpath expr="//img" position="attributes">