Remove JSON.php fallback (#2528)

After moving to PHP 5.6+ https://github.com/FreshRSS/FreshRSS/pull/2527
it should not be necessary to have the JSON.php fallback anymore, which
was mainly there due to an obscure licensing issue 6 years ago in Debian
https://wiki.debian.org/qa.debian.org/jsonevil , which broke Ubuntu
13.10 https://github.com/FreshRSS/FreshRSS/issues/306

phpcs.xml

@@ -8,7 +8,6 @@
 	<exclude-pattern>./lib/SimplePie/</exclude-pattern>
 	<exclude-pattern>./lib/PHPMailer/</exclude-pattern>
 	<exclude-pattern>./lib/http-conditional.php</exclude-pattern>
-	<exclude-pattern>./lib/JSON.php</exclude-pattern>
 	<exclude-pattern>./lib/lib_phpQuery.php</exclude-pattern>
 	<!-- Duplicate class names are not allowed -->
 	<rule ref="Generic.Classes.DuplicateClassName"/>