remove reading data provider username and password from file

This reverts #1455 because the contributor cannot sign the CLA

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

sftpgo.json

@@ -198,9 +198,7 @@
     "host": "",
     "port": 0,
     "username": "",
-    "username_file": "",
     "password": "",
-    "password_file": "",
     "sslmode": 0,
     "disable_sni": false,
     "target_session_attrs": "",