Delete a user from Opal internal users registry. Fails silently if user does not exist.
Examples
if (FALSE) { # \dontrun{
o <- opal.login('administrator','password', url='https://opal-demo.obiba.org')
pwd <- oadmin.user_add(o, "foo", groups = c("datashield", "CNSIM"))
oadmin.user_delete(o, "foo")
opal.logout(o)
} # }