Remove the adminpack contrib extension

Remove the adminpack contrib extension

The adminpack extension was only used to support pgAdmin III,  which
in turn was declared EOL many years ago. Removing the extension also
allows us to remove functions from core as well which were only used
to support old version of adminpack.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CALj2ACUmL5TraYBUBqDZBi1C+Re8_=SekqGYqYprj_W8wygQ8w@mail.gmail.com