Closes #537: Be lenient to extensions operating on snippet fields

* yasnippet.el (yas--on-protection-overlay-modification): Commit the
snippets being destroyed instead of signalling a protection violation.
Clean up previous change

* yasnippet.el: Update comment.
(yas--protection-violation): Removed.
(yas--on-protection-overlay-modification): Simplified.
(yas--post-command-handler): No longer reference
`yas--protection-violation'