Bug 1268738 - Remove the background image from devtools breadcrumbs in the firebug theme. r=ntim

Prevents two breadcrumb arrows from appearing because the light-theme breadcrumb
is a background while the firebug-theme one is a CSS generated element

MozReview-Commit-ID: 20DEHid7lOP

--HG--
extra : transplant_source : kh%A0%F2%14%0B%FF%F3%BC%A6%28%1E%A39%9E%F2F%04%15%7B

devtools/client/themes/widgets.css

@@ -351,6 +351,7 @@
 .theme-firebug .breadcrumbs-widget-item {
   margin-inline-start: 10px;
   margin-inline-end: 1px;
+  background-image: none;
   border: 1px solid transparent;
   color: #141414;
   border-radius: 2px;