org-persist.el: Silence fewer warnings and fix found warning

org-persist.el: Silence fewer warnings and fix found warning

* lisp/org/org-persist.el: Minor cleanup.
(org-persist-directory, org-persist-remote-files)
(org-persist-default-expiry): Remove redundant `:group` arg.
(org-persist-collection-let): Document lack of hygiene.
Remove `with-no-warnings`.
(org-persist--find-index): Reindent, and simplify initialization of `r`.
(org-persist-associated-files:generic): Move before first use.

(cherry picked from commit 3998242664604e9e91830b10529cc2da6bd82d8e)