Mash cleanup: remove more methods from mojom::LoginScreen.

ShowErrorMessage and ClearErrors are deleted, as they've remained
unimplemented for over a year.

SetShowGuestButtonInOobe and SetShowParentAccessButton are renamed
and moved to LoginScreen.

SetShowParentAccessDialog is removed as part of the public API
since it is never called from Chrome.

TBR=tsepez@chromium.org

Bug: 958206
Change-Id: Ibde235d3dd1013a33855874247231613d0fd4da9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639459
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665577}

• ash/login/login_screen_controller.cc[diff]
• ash/login/login_screen_controller.h[diff]
• ash/login/ui/lock_contents_view.cc[diff]
• ash/login/ui/lock_contents_view.h[diff]
• ash/login/ui/lock_contents_view_unittest.cc[diff]
• ash/login/ui/lock_debug_view.cc[diff]
• ash/login/ui/lock_screen.cc[diff]
• ash/login/ui/lock_screen.h[diff]
• ash/login/ui/login_data_dispatcher.cc[diff]
• ash/login/ui/login_data_dispatcher.h[diff]
• ash/public/cpp/login_screen.h[diff]
• ash/public/interfaces/login_screen.mojom[diff]
• ash/shelf/login_shelf_view.cc[diff]
• ash/shelf/login_shelf_view.h[diff]
• ash/shelf/login_shelf_view_unittest.cc[diff]
• chrome/browser/chromeos/child_accounts/screen_time_controller.cc[diff]
• chrome/browser/chromeos/login/lock/views_screen_locker.cc[diff]
• chrome/browser/chromeos/login/wizard_controller.cc[diff]
• chrome/browser/ui/ash/test_login_screen.cc[diff]
• chrome/browser/ui/ash/test_login_screen.h[diff]
• chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc[diff]
• chrome/browser/ui/webui/chromeos/login/supervision_transition_screen_handler.cc[diff]