[ticket/16084] Remove redundant backup radio button

[ghost]

PHPBB3-16084

Checklist:

• Correct branch: master for new features; 3.3.x & 3.2.x for fixes
• Tests pass
• Code follows coding guidelines: master, 3.3.x and 3.2.x
• Commit follows commit message format

Tracker ticket (set the ticket ID to your ticket ID):

https://tracker.phpbb.com/browse/PHPBB3-16084

[ghost]

Just to clarify, the ticket suggested

Better if just text explaining that the backup is stored in /stores folder.

But this is already present in the module description:

Here you can backup all your phpBB related data. The resulting archive will be stored in your store/ folder. Depending on your server configuration you may be able to compress the file in a number of formats.