Remove the "Replace FOP" configuration option from the PDF Export administration section

We're going to deprecate the old FOP-based PDF export, so we should stop exposing the "Replace FOP" configuration option in the PDF Export administration section. We can keep the configuration in xwiki.properties for a while, but we should drop that too eventually.

See https://forum.xwiki.org/t/deprecate-old-pdf-export-and-retire-it/13520/11 .