Remove BuiltInChromeOsApps::SetHideSettingsAppForTesting().

This CL removes a testing hack which is no longer used.

Bug: None
Change-Id: I68f1d84593ddb6fe45df52500eaeb4fd0767dd56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2400651


Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805630}