Cannot Uninstall Purchased Theme, Porto
You can't uninstall the default theme, because there wouldn't be a theme left for the site. Similarly you can't uninstall the Seven theme when it's selected as the admin theme.

You need to change the default to another theme, then under normal circumstances the "Uninstall" link will appear where you'd expect it.

If it doesn't, consult the documentation or contact support for the theme seller, because they've done something non-standard, and it's not likely to be possible to guess what that might have been from afar (especially if it was done as part of the initial SQL import)