Remove temporary using declarations from c/b/ash/a*

Those were added to avoid code churn in the migration of Ash-only files
to namespace ash. Now that the migration is almost finished, this code
can be removed.

Also remove unneeded ash:: prefixes.

This CL is part of the ChromeOS source code directory migration:
https://docs.google.com/document/d/1g-98HpzA8XcoGBWUv1gQNr4rbnD5yfvbtYZyPDDbkaE.

Bug: 1164001
Change-Id: I4cba65f1b9db2d5d1945e574d8c6d8bb6d1edf14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4008280
Auto-Submit: Henrique Ferreiro <[email protected]>
Owners-Override: James Cook <[email protected]>
Commit-Queue: Henrique Ferreiro <[email protected]>
Reviewed-by: James Cook <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1070502}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index b0fd561c..d0f06da 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1141,7 +1141,7 @@
 #if BUILDFLAG(IS_CHROMEOS_ASH)
   arc::prefs::RegisterLocalStatePrefs(registry);
   ChromeOSMetricsProvider::RegisterPrefs(registry);
-  chromeos::ArcKioskAppManager::RegisterPrefs(registry);
+  ash::ArcKioskAppManager::RegisterPrefs(registry);
   ash::AudioDevicesPrefHandlerImpl::RegisterPrefs(registry);
   ash::cert_provisioning::RegisterLocalStatePrefs(registry);
   ash::CellularESimProfileHandlerImpl::RegisterLocalStatePrefs(registry);