SDelete with session_persistence

Thanks for vim-startify! I've noticed that if I have `let g:startify_session_persistence = 1`, load a session, `:SDelete` it, and then quit vim, the session is recreated - presumably because vim-startify is keeping track of the session, even after the delete, and session persistence is saving when I quit.

Reproduce the reported sequence in Vim: enable session persistence, load a session, run :SDelete, and quit. Trace how vim-startify tracks the deleted session and saves sessions on exit; done means the deleted session is not recreated after quitting.

Written by the indexing model from the issue text.