[5.0] Remove IE9 code, don't force download of JSON documents

[SharkyKZ]

Summary of Changes

Removes some IE9 code which is causing modern browsers to render JSON documents as plaintext. Also removes forcing file download, which wasn't working anyway because of the first part of code.

Testing Instructions

Enter a JSON page, e.g. index.php?option=com_ajax&format=json. Confirm that JSON is rendered in the browser.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[laoneo]

This surely needs to be documented.

[toivo]

I have tested this item ✅ successfully on ba431a9

Tested successfully in Joomla 5.0.0-dev of 14 March using PHP 8.1.10.

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[HLeithner]

thanks