[#2093] trim irodsctl.pl functionality

The irodsctl.pl script still listed functionality that is no longer
supported in iRODS 4.0+.

iRODS will no longer manage the installation of a database.  The
functions and features removed here were no longer true.  The usage
information has been updated for "irodsctl -h".

This edit just brings the script inline with the practical usage
pattern defined with 4.0+ (all databases are 'external' to iRODS).