s3:idmap_tdb2: remove the undocumented option of the silly name "tdb:idmap2.tdb"

s3:idmap_tdb2: remove the undocumented option of the silly name "tdb:idmap2.tdb"

In ancient times, when ctdb had not support for persistent databases and
tdb2 was introduced as a two-layer solution and it was more important than
today to be able to change the location of the permanent database file
because it had to reside on shared storage.

But these were times when idmap_tdb2 was not even officially released.
Nowadays, with ctdb handling the persistent idmap2.tdb database, the path
is stripped anyways, so this undocumented option has become unnecessary
and is hence removed.

Autobuild-User: Michael Adam <obnox@samba.org>
Autobuild-Date: Wed Jul 27 05:37:57 CEST 2011 on sn-devel-104