Remove special selector to see if tests pass on CI (#5362)

Author: darkwing

src/test/mochitest/browser_dbg-sources.js

@@ -33,7 +33,7 @@ add_task(async function() {
   await clickElement(dbg, "sourceArrow", 2);
 
   await assertSourceCount(dbg, 7);
-  await clickElement(dbg, "sourceDirectory", 3);
+  await clickElement(dbg, "sourceArrow", 3);
   await assertSourceCount(dbg, 8);
 
   // Select a source

src/test/mochitest/head.js

@@ -940,7 +940,6 @@ const selectors = {
   sourceNode: i => `.sources-list .tree-node:nth-child(${i}) .node`,
   sourceNodes: ".sources-list .tree-node",
   sourceArrow: i => `.sources-list .tree-node:nth-child(${i}) .arrow`,
-  sourceDirectory: i => `.sources-list .tree-node:nth-child(${i})`,
   resultItems: ".result-list .result-item",
   fileMatch: ".managed-tree .result",
   popup: ".popover",