Don't show favicon to prevent iteration through subfolders

The codepath for generating the favicons iterates through subnodes and if one of those nodes is unavailable is throwing a 503 exception. Since these favicons don't have any use except of "making a tool for developers looking nicer" I consider it feasible to remove them.
Merge pull request #12448 from owncloud/no503forRoot

Don't show favicon to prevent iteration through subfolders