Remove Support for On-Disk JSON mappings, convert existing

[fm3]

It is tedious to support all kinds of features for both json and agglomerate-file based mappings.

On-Disk Json mappings aren’t really used anymore. We should find and convert the existing ones and then remove this feature.

Note that some of the front-end code dealing with json mappings will probably survive, as it is also used in merger-mode and proofreading.