chore(auth): Remove credential program leftovers

With the OAuth2 flow being default now the logic to fetch login credentials via
external programs (i.e. `pass`) has become obsolote.

This change removes documentation/config leftovers.

src/config.rs

@@ -100,17 +100,9 @@ pub struct ConfigValues {
     pub statusbar_format: Option<String>,
     pub library_tabs: Option<Vec<LibraryTab>>,
     pub hide_display_names: Option<bool>,
-    pub credentials: Option<Credentials>,
     pub ap_port: Option<u16>,
 }
 
-/// Commands used to obtain user credentials automatically.
-#[derive(Serialize, Deserialize, Debug, Default, Clone)]
-pub struct Credentials {
-    pub username_cmd: Option<String>,
-    pub password_cmd: Option<String>,
-}
-
 /// The ncspot theme.
 #[derive(Serialize, Deserialize, Debug, Default, Clone)]
 pub struct ConfigTheme {