log: remove redundant "timestamp" label from default template

We now already have the "timestamp" label from the auto-labeling of
method calls.

Without the use of the "email" label in the previous commit, we would
now emit an unnecessary color code when the "author" label is added
but before the "timestamp" is added. So that patch wasn't just about
simplifiying the config :)

src/commands.rs

@@ -1561,7 +1561,7 @@ fn log_template(settings: &UserSettings) -> String {
             change_id.short()
             if(divergent, label("divergent", " divergent"))
             " " author.email()
-            " " label("timestamp", {committer_timestamp})
+            " " {committer_timestamp}
             if(branches, " " branches)
             if(tags, " " tags)
             if(working_copies, " " working_copies)