Remove deprecated 'json' module

Remove deprecated 'json' module

This is an empty placeholder module. It was kept for backwards compatibility
with older extensions that were still supporting MediaWiki versions that in
turn supported older browsers that in turn did not yet implement the JSON
interface of the ES5 standard.

There is no longer any use of 'json' module anywhere in Wikimedia Git,
nor anywhere found by Codesearch, nor anywhere on-wiki in gadgets at WMF.

Bug: T127328
Change-Id: I8ba40a73dc900909e3fa3bd3ebe88616c9a26d3c