Markdown: Remove usage of $HTTP_RAW_POST_DATA

[kraftbj]

Global variable '$HTTP_RAW_POST_DATA' is deprecated since PHP 5.6 and removed since PHP 7.0; Use php://input instead is the final issue left to resolve #4346, which itself is the final step to resolve.

I'm opening this as a separate issue so we can close out 4346 to focus on the last actionable item from that conversation.

The only usage is all within one function within /modules/markdown/easy-markdown.php.

[wigglemuff]

The only usage is all within one function within /modules/markdown/easy-markdown.php.

This seems to be fixed in here: #9905

Here's a comment by Igor on the specific changes relevant to this issue:
#9905 (comment)

I'm closing this issue for now! If I'm wrong, reopen and let me know :)