portable mode: remove support for services discovery via multicast DNS

The library used for mDNS doesn't seem well maintained and I think this
feature is rarely used

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

docs/portable-mode.md

@@ -15,12 +15,6 @@ Usage:
   sftpgo portable [flags]
 
 Flags:
-  -C, --advertise-credentials           If the SFTP/FTP service is
-                                        advertised via multicast DNS, this
-                                        flag allows to put username/password
-                                        inside the advertised TXT record
-  -S, --advertise-service               Advertise configured services using
-                                        multicast DNS
       --allowed-patterns stringArray    Allowed file patterns case insensitive.
                                         The format is:
                                         /dir::pattern1,pattern2.
@@ -148,15 +142,3 @@ Flags:
       --webdav-port int                 0 means a random unprivileged port,
                                         < 0 disabled (default -1)
 ```
-
-In portable mode, SFTPGo can advertise the SFTP/FTP services and, optionally, the credentials via multicast DNS, so there is a standard way to discover the service and to automatically connect to it.
-
-Here is an example of the advertised SFTP service including credentials as seen using `avahi-browse`:
-
-```console
-= enp0s31f6 IPv4 SFTPGo portable 53705                         SFTP File Transfer   local
-   hostname = [p1.local]
-   address = [192.168.1.230]
-   port = [53705]
-   txt = ["password=EWOo6pJe" "user=user" "version=0.9.3-dev-b409523-dirty-2019-10-26T13:43:32Z"]
-```