We were changing the user's user_option.bookmark_auto_delete_preference
to whatever they changed it to in the bookmark modal to use as default
for future bookmarks. However this was leading to a lot of confusion
since if you wanted to set it for one bookmark you had to remember to
change it back on the next one.
This commit removes that automatic functionality, and instead moves
the bookmark auto delete preference to User Preferences > Interface
in an explicit dropdown.
