[signin] Remove unused ChromeOS signin code

Following files are no longer built on ChromeOS:
c/b/ui/bookmarks/bookmark_bubble_sign_in_delegate.cc
c/b/ui/bookmarks/bookmark_bubble_sign_in_delegate.h
c/b/ui/views/bookmarks/bookmark_bubble_sign_in_delegate_browsertest.cc
c/b/ui/views/sync/bubble_sync_promo_view_unittest.cc
c/b/ui/views/sync/bubble_sync_promo_view.cc
c/b/ui/views/sync/bubble_sync_promo_view.h

Dead ChromeOS code is removed from c/b/signin/

Some duplicated code is refactored into:
chrome/browser/ui/views/sync/bubble_sync_promo_view_util.h

Change-Id: I30eee77bdf856282e50a5315944c606559e41047
Reviewed-on: https://chromium-review.googlesource.com/c/1445940
Reviewed-by: Dana Fried <dfried@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Auto-Submit: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629908}
22 files changed