Coding Standards: Remove unnecessary isset() from xmlrpc.php.

This removes a redundant `isset( $HTTP_RAW_POST_DATA )` from `xmlrpc.php`, as the variable is already set in the code block immediately preceding the affected line.

Follow-up to [3498], [5445], [47926].

Props justlevine.
See #52217.

git-svn-id: https://develop.svn.wordpress.org/trunk@59376 602fd350-edb4-49c9-b593-d223f7449a82

src/xmlrpc.php

@@ -22,9 +22,7 @@
 }
 
 // Fix for mozBlog and other cases where '<?xml' isn't on the very first line.
-if ( isset( $HTTP_RAW_POST_DATA ) ) {
-	$HTTP_RAW_POST_DATA = trim( $HTTP_RAW_POST_DATA );
-}
+$HTTP_RAW_POST_DATA = trim( $HTTP_RAW_POST_DATA );
 // phpcs:enable
 
 /** Include the bootstrap for setting up WordPress environment */