Somewhat related to / duplicate of this issue: http://wordpress.org/support/topic/plugin-contact-form-7-3rd-party-integration-contact-form-7-modules-hidden-fields-update-not-applied
However, I think you're asking something slightly different -- you want to know if we can just get rid of the Hidden Fields plugin entirely, because it appears that it's not in use.
If that is the question, then the simple answer is yes, you don't need the "hidden fields" plugin.
But you'd be under a misunderstanding -- the plugin is technically active even though it doesn't show up as being so. As I mention in the other post, the plugin code is directly included, and thus available regardless of how the plugin page shows it (you can prove this by creating a contact form and looking under "Generate Tag" - there will be a "hidden field" option). Showing up as "active/inactive" is triggered by the presence of the plugin comment header.
As an aside, I originally included the plugin as an easier way to specify static value fields (before I provided the checkbox "Is Value"). The intention is that you can create multiple forms using the same "3rdparty service", like Bronto, which has a unique identifier for
each form. Instead of creating multiple "services" which all have identical field mappings and only differ in their static values, it made sense to specify the unique options directly as hidden fields of the form itself.
Because this issue is so confusing, and because this "feature" is not intrinsically needed by the plugin (and I'm guessing no one is using it), I would say it makes sense to remove the extra plugin. In the near future, if no-one comes across this and mentions that they need it, I'll update the plugin to take it out and indicate how you could include the plugin separately to accomplish the task.
For now, if the message bothers you you may:
- delete the contents of
hidden.php (not the file itself, or you may get an "include file not found" error)
- remove the plugin header code from
hidden.php, which will prevent it from showing up in the list (but then you might forget that you have that functionality present)
- Change the version number in the plugin header code of
hidden.php to something like
9999Fork.1.2, which will trick WP into thinking your current version is newer than any possible upgrades, thus "preventing" the ugrade notice.