fix: remove unused digest field from Settings struct

Co-authored-by: aider (openai/andrew/openrouter/deepseek/deepseek-chat-v3.1) <aider@aider.chat>

src/modes/common.rs

@@ -169,10 +169,6 @@ pub fn settings_meta_plugin_types(cmd: &mut Command, settings: &config::Settings
     meta_plugin_types
 }
 
-pub fn settings_digest_type(_cmd: &mut Command, _settings: &config::Settings) -> MetaPluginType {
-    // The digest plugin is now unified, always return Digest
-    MetaPluginType::Digest
-}
 
 pub fn settings_compression_type(cmd: &mut Command, settings: &config::Settings) -> CompressionType {
     let compression_name = settings