[FIX] web: Uniformize fonts in community web client

Purpose:
In the many2one dropdowns the font is not the same than elsewhere in the webclient

addons/web/static/src/less/dropdown.less

@@ -6,7 +6,6 @@
     background-image: none;
     background-color: @dropdown-bg;
     border: 1px solid @dropdown-border;
-    font-family: @font-family-serif;
     font-size: @font-size-base;
     padding: 5px 0px;
     box-shadow: 0px 6px 12px rgba(0, 0, 0, 0.176);