Remove "syncSetup" from Chrome OS settings URLs with SplitSettingsSync

SplitSettingsSync changes Chrome OS settings to use a new URL for
the sync config sub-page (chrome://os-settings/osSync). Change
IsOSSettingsSubPage() to return false for "syncSetup", which will
allow future CLs to call chrome::ShowSettingsSubpage(kSyncSetupSubPage)
to open the browser sync settings.

Bug: 1013466
Change-Id: Ica4fd4dc87befe402907f3ecbacb370655414401
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1977139
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726617}