General: More PHP compatibility adjustments.

In this round:
- Removed references to `safe_mode` in `class-php3.php`. This was removed in PHP 5.4.
- Add inline exclude comments for compatibility checks in getID3.

Follow up of [47735-47736].

See #49922.
Built from https://develop.svn.wordpress.org/trunk@47737


git-svn-id: https://core.svn.wordpress.org/trunk@47513 1a063a9b-81f0-0310-95a4-ce76da25c4cd