Finish moving a couple commands to plugins, remove unused plugin

src/plugins/sum.rs

@@ -1,48 +0,0 @@
-use indexmap::IndexMap;
-use nu::{serve_plugin, Args, CommandConfig, Plugin, Primitive, ShellError, Value};
-
-struct Sum;
-
-impl Sum {
-    fn new() -> Sum {
-        Sum
-    }
-}
-
-impl Plugin for Sum {
-    fn config(&mut self) -> Result<CommandConfig, ShellError> {
-        Ok(CommandConfig {
-            name: "sum".to_string(),
-            mandatory_positional: vec![],
-            optional_positional: vec![],
-            can_load: vec![],
-            can_save: vec![],
-            is_filter: false,
-            is_sink: true,
-            named: IndexMap::new(),
-            rest_positional: true,
-        })
-    }
-
-    fn sink(&mut self, _args: Args, input: Vec<Value>) {
-        let mut total = 0i64;
-
-        for v in input {
-            match v {
-                Value::Primitive(Primitive::Int(i)) => {
-                    total += i;
-                }
-                Value::Primitive(Primitive::Bytes(i)) => {
-                    total += i as i64;
-                }
-                _ => {}
-            }
-        }
-
-        println!("Result: {}", total);
-    }
-}
-
-fn main() {
-    serve_plugin(&mut Sum::new());
-}