whitespace in exports

[aristath]

The issue: extra spacing gets added before elements. This causes things like title attributes in menus to break, content build using a page-builder to stop working etc.

This was done is this commit which broke the export functionality for us - or rather it broke content that gets imported on a site if said content has been exported using WP 5.1.1: https://core.trac.wordpress.org/changeset?old_path=%2Ftags%2F5.0.3%2Fsrc%2Fwp-admin%2Fincludes%2Fexport.php&old=45013&new_path=%2Ftags%2F5.1.1%2Fsrc%2Fwp-admin%2Fincludes%2Fexport.php&new=45013&sfp_email=&sfph_mail=#file0

This was probably a coding-standards "fix" so things like this:
<title><?php
were replaced, the opening PHP tag went to the line below and indented. That indentation messes up things.

[aristath]

This is a bug introduced in 5.1.1 and reverting the commit fixes the issue, so that is the simplest and fastest way to handle this. I'm not attaching a patch, if we don't want to revert the commit there are ways to fix it but not as simple or fast.

[sgastard]

Duplicate of #46575.

Duplicate of https://core.trac.wordpress.org/ticket/46575