Bug 11891: (follow-up) remove custom storage class

Bug 11891: (follow-up) remove custom storage class

For now, just rely on the normal DBIx::Class cleanup handler;
otherwise, the following warning is displayed in logs:

  (in cleanup) Can't locate object method "_preserve_foreign_dbh" via package "Koha::Storage" at .../Koha/Storage.pm

Using the default DBIC storage class also means that we
don't have to manually do things like set the SQL limit dialect;
meaning that we don't get the following warning message:

  DBIx::Class::ResultSet::find(): Your storage class (Koha::Storage) does
  not set sql_limit_dialect and you have not supplied an explicit
  limit_dialect in your connection_info. DBIC will attempt to use the
  GenericSubQ dialect, which works on most databases but can be (and
  often is) painfully slow. Please file an RT ticket against
  'Koha::Storage' . at /home/gmc/koha/C4/Members.pm line 840

Test plan: same as main patch

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>