drop pdo_ibm driver support

UPGRADE.md

 # Upgrade to 2.5
 
-No BC breaks yet.
+## Support for pdo_ibm driver removed
+
+The ``pdo_ibm`` driver is buggy and does not work well with Doctrine. Therefore it will no
+longer be supported and has been removed from the ``Doctrine\DBAL\DriverManager`` drivers
+map. It is highly encouraged to to use `ibm_db2` driver instead if you want to connect
+to an IBM DB2 database as it is much more stable and secure.
+
+If for some reason you have to utilize the ``pdo_ibm`` driver you can still use the `driverClass`
+connection parameter to explicitly specify the ``Doctrine\DBAL\Driver\PDOIbm\Driver`` class.
+However be aware that you are doing this at your own risk and it will not be guaranteed that
+Doctrine will work as expected.
 
 # Upgrade to 2.4
 

lib/Doctrine/DBAL/DriverManager.php

@@ -44,7 +44,6 @@ final class DriverManager
          'pdo_oci'            => 'Doctrine\DBAL\Driver\PDOOracle\Driver',
          'oci8'               => 'Doctrine\DBAL\Driver\OCI8\Driver',
          'ibm_db2'            => 'Doctrine\DBAL\Driver\IBMDB2\DB2Driver',
-         'pdo_ibm'            => 'Doctrine\DBAL\Driver\PDOIbm\Driver',
          'pdo_sqlsrv'         => 'Doctrine\DBAL\Driver\PDOSqlsrv\Driver',
          'mysqli'             => 'Doctrine\DBAL\Driver\Mysqli\Driver',
          'drizzle_pdo_mysql'  => 'Doctrine\DBAL\Driver\DrizzlePDOMySql\Driver',
@@ -73,7 +72,6 @@ final class DriverManager
      *     pdo_pgsql
      *     pdo_oci (unstable)
      *     pdo_sqlsrv
-     *     pdo_ibm (unstable)
      *     pdo_sqlsrv
      *     mysqli
      *     sqlanywhere