fix(modem): Cleanup unused configs from PPPoS example

Merges #403

components/esp_modem/examples/pppos_client/main/Kconfig.projbuild

@@ -63,26 +63,6 @@ menu "Example Configuration"
         help
             Set APN (Access Point Name), a logical name to choose data network
 
-    config EXAMPLE_MODEM_PPP_AUTH_USERNAME
-        string "Set username for authentication"
-        default "espressif"
-        depends on !EXAMPLE_MODEM_PPP_AUTH_NONE
-        help
-            Set username for PPP Authentication.
-
-    config EXAMPLE_MODEM_PPP_AUTH_PASSWORD
-        string "Set password for authentication"
-        default "esp32"
-        depends on !EXAMPLE_MODEM_PPP_AUTH_NONE
-        help
-            Set password for PPP Authentication.
-
-    config EXAMPLE_MODEM_PPP_AUTH_NONE
-        bool "Skip PPP authentication"
-        default n
-        help
-            Set to true for the PPP client to skip authentication
-
     config EXAMPLE_SEND_MSG
         bool "Short message (SMS)"
         default n