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.

doc/users.md

@@ -459,17 +459,3 @@ The credentials are stored in `librespot/credentials.json` in the user's cache d
 
 The `logout` command can be used to remove cached credentials. See
 [Vim-Like Commands](#vim-like-commands).
-
-### Using a Password Manager
-If you would like ncspot to retrieve your login data from command results,
-i.e. because you use a password manager like `pass`, you can add the following
-configuration:
-
-```toml
-[credentials]
-username_cmd = "echo username"
-password_cmd = "pass spotify.com/username"
-```
-
-Do note that this is only required for the initial login or when your credential
-token has expired.

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 {