purge: Remove REQUEST_URI fallback

No longer needed and masked an expected error when "uri" is unset.

Author: Krinkle

purge.php

@@ -36,7 +36,7 @@
 $purgeSite = getenv( 'PURGE_SITE' ) ?: @$_GET['site'] ?: 'releases';
 $striketrackerPurgeHostname = @$purgeHostnames[$purgeSite] ?: false;
 
-$purgeUri = getenv( 'PURGE_URI' ) ?: @$_GET['uri'] ?: @$_SERVER['REQUEST_URI'] ?: false;
+$purgeUri = getenv( 'PURGE_URI' ) ?: @$_GET['uri'] ?: false;
 
 if ( !$striketrackerUrl
 	|| !$striketrackerToken