Export of custom profiles fields lacks visibility check

https://tracker.moodle.org/browse/CONTRIB-6807 has added custom profile fields to the Scheduler, export, but the select boxes for which fields to export includes options which are marked 'Visible to user' only, or to no one (except Admins).

The export itself correctly checks visibility, so the teacher ends up with an export file containing empty columns for these fields. Depending on what these fields are, and how they are named, this may however be confusing for the teacher. For example they may believe that they should be able to see the data (but cannot), or they may be concerned that data will be revealed to them that they should not see.

I've attached a small patch which pinpoints the lines that are an issue and fixes them, but my approach is very much a 'quick and dirty' solution. Someone more familiar with the plugin could certainly improve on this.

Steps to reproduce:

• Create a custom profile field marked as 'Visible to user' or 'Not visible'
• Login as a teacher and go to a Scheduler export page
• Expand the 'Data to include' heading

Expected result:

• The newly-created field will not be shown, because the teacher has no permission to export its contents

What actually happens:

• The newly-created field is included in the list of options
• If the teacher selects this field for export, they receive an empty column in the exported file.