cli: merge 'fields' with 'info' command

Update the 'info' command to handle 'fields' as well; remove
fields.

mu/mu-cmd.hh

@@ -79,15 +79,6 @@ Result<void> mu_cmd_cfind(const Store& store, const Options& opts);
  */
 Result<void> mu_cmd_extract(const Options& opts);
 
-/**
- * execute the 'fields' command
- *
- * @param opts configuration options
- *
- * @return Ok() or some error
- */
-Result<void> mu_cmd_fields(const Options& opts);
-
 /**
  * execute the 'find' command
  *