It is recommended that you avoid declaring a character set in a meta tag because that disables the lookahead downloader in Internet Explorer 8 and that can slow down the Joomla page load time for people using that browser.

To remove the <meta http-equip> tag in Joomla 2.5.6 comment out (put 2 forward slashes at the beginning of) lines 72-102 in the file libraries/joomla/document/html/renderer/head.php  (see below)

Also, remember to declare a character set elsewhere - such as in your .htaccess file as described here.

// Generate META tags (needs to happen as early as possible in the head)
//foreach ($document->_metaTags as $type => $tag)
//{
//foreach ($tag as $name => $content)
//{
//if ($type == 'http-equiv')
//{
//$content .= '; charset=' . $document->getCharset();
//$buffer .= $tab . '<meta http-equiv="' . $name . '" content="' . htmlspecialchars($content) . '" />' . $lnEnd;
//}
//elseif ($type == 'standard' && !empty($content))
//{
//$buffer .= $tab . '<meta name="' . $name . '" content="' . htmlspecialchars($content) . '" />' . $lnEnd;
//}
//}
//}

// Don't add empty descriptions
//$documentDescription = $document->getDescription();
//if ($documentDescription)
//{
//$buffer .= $tab . '<meta name="description" content="' . htmlspecialchars($documentDescription) . '" />' . $lnEnd;
//}

// Don't add empty generators
//$generator = $document->getGenerator();
//if ($generator)
//{
//$buffer .= $tab . '<meta name="generator" content="' . htmlspecialchars($generator) . '" />' . $lnEnd;
//}

//$buffer .= $tab . '<title>' . htmlspecialchars($document->getTitle(), ENT_COMPAT, 'UTF-8') . '</title>' . $lnEnd;