Remove fixDefaultJsonContentPages.php

Why:

• The fixDefaultJsonContentPages.php maintenance script was added in 2986d47c9052af08e37b363ad614e32f5ccff508 which was MW 1.27
• Per the version policy, wikis wishing to upgrade to MW 1.43 should upgrade via 1.35 or 1.39 before moving to 1.43.
• As such, this script will have been already run for any wiki upgrading to 1.43 and therefore this script is unused.
• Removing the script is useful to reduce the amount of untested code in the maintenance directory and reducing unnecessary maintenance on now unused code.

What:

• Remove fixDefaultJsonContentPages.php

Bug: T373335
Change-Id: Ie20f55c6a8723573aa7e9acd67766af9dfb67269