Revert "added to the background object to the Boards collection."

This reverts commit 7de25da94c2ccc806ec967dc73a863617a7ecef9.

collections/boards.js

@@ -14,9 +14,6 @@ Boards.attachSchema(new SimpleSchema({
     type: Date,
     denyUpdate: true
   },
-  background: {
-    type: Object
-  },
   // XXX Inconsistent field naming
   modifiedAt: {
     type: Date,