[Extensions cleanup] Remove unused ScopedTestDialogAutoConfirm methods

One of ScopedTestDialogAutoConfirm constructors and GetOptionSelected()
are not longer used, so we can remove them. With that, we can also
remove g_extension_dialog_option_to_select

Bug: 332587373
Change-Id: Ief764fe8231f3b36198f968f141e1ee8484e1ac1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5416996


Commit-Queue: David Bertoni <dbertoni@chromium.org>
Auto-Submit: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281946}