log: remove unused and inconsistent log label

We add a top-level `log` label to the output from `jj log`, but we
never define any colors based on it. More importantly, it was
inconsistent between the graph and non-graph cases. When showing the
graph, any colors set based on it would only apply to the graph itself
[1] because we use a separate formatter for each commit in the
graphlog and that formatter didn't inherit the `log` label. So let's
just remove the label for now. We can consider adding it back for each
commit later. That's what we do for `jj op log`, but it's simpler in
that case because it doens't have a `--no-graph` version.

 [1] Well, mostly; it would also apply to any uncolored element
     immediately right of the graph.

src/commands/mod.rs

@@ -1444,7 +1444,6 @@ fn cmd_log(ui: &mut Ui, command: &CommandHelper, args: &LogArgs) -> Result<(), C
         ui.request_pager();
         let mut formatter = ui.stdout_formatter();
         let mut formatter = formatter.as_mut();
-        formatter.push_label("log")?;
 
         if !args.no_graph {
             let mut graph = get_graphlog(command.settings(), &mut formatter);