Mssql: Remove freetds from supported driver list

There's no mention of this identifier on
https://www.php.net/manual/en/ref.pdo-dblib.connection.php.

I also found no mention of it at any time
(using the wayback machine)

So there's only Zend's v1 adapter which uses it as a fallback
for `sybase`, which implies it being only an alias. Since we
are comparing this with the result of `PDO::getAvailableDrivers()`
and don't use it as configuration option, I highly doubt we
retrieve aliases there.

src/Adapter/Mssql.php

@@ -16,7 +16,7 @@ class Mssql extends BaseAdapter
 
     public function getDsn(Config $config)
     {
-        $drivers = array_intersect(['dblib', 'mssql', 'sybase', 'freetds'], PDO::getAvailableDrivers());
+        $drivers = array_intersect(['dblib', 'mssql', 'sybase'], PDO::getAvailableDrivers());
 
         if (empty($drivers)) {
             throw new RuntimeException('No PDO driver available for connecting to a Microsoft SQL Server');