Removing archived wal files on Master

Hi,
I recently acquired a legacy server. This is running Postgresql 9.3 on Ubuntu 16.04.
There is around 200GB worth of archived wal files (~12,500 files) located on /var/lib/postgresql/9.3/main/archive.
I have checked and can confirm that this is a standalone server without any Replication setup and no secondary/slave server talking to it.

Checking the /etc/postgresql/9.3/main/postgresql.conf file (write ahead log section), shows the following:
wal_level = minimal
archive_mode = off
archive_command = 'test ! -f /var/lib/postgresql/9.3/main/archive/%f && cp -i %p /var/lib/postgresql/9.3/main/archive/%f </dev/null'

Replication section shows all default values.

Could you please advice how I can cleanup all of these 200GB worth of files.
I've searched through numerous postgresql books/blogs/articles which all have very good advise on how to setup wal archiving but unfortunately not on how to disable it.

Kind regards,
Michael