Cleanup social network icons #3542

The social network icons I added in #2791 could use some cleaning up.

(In [7524]) Fixes #3542 Updating logos to latest from the sites including transparent background

I used this code to download:
require_once PIWIK_INCLUDE_PATH . '/core/DataFiles/Socials.php';
foreach($GLOBALS['Piwik_socialUrl'] as $url => $name) {
$destinationPath = PIWIK_USER_PATH . '/plugins/Referers/images/socials/'.$url.'.png';
$url = 'http://g.etfv.co/http://'. $url;
echo "$url in $destinationPath <br>";
Piwik_Http::sendHttpRequest($url, 10, 'Mozilla', $destinationPath);
//break;
}

and then something like mogrify -format png *.ico