The watch-later mechanism has always unconditionally wrote start to
files to save the playback position. When this was later expanded to
watch-later-options, this logic was kept in place. But we don't actually
have to unconditionally write this and can allow users to remove the
option from the list if they want to. The start value still requires
some special handling; it should always be written if possible
regardless of the value changing. However, we can just place it within
the default set of options for watch-later-options so it can be removed
like any other.
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