dashboard: remove the dashboard (#20279)

This removes the dashboard project. The dashboard was an experimental
browser UI for geth which displayed metrics and chain information in
real time. We are removing it because it has marginal utility and nobody
on the team can maintain it.

Removing the dashboard removes a lot of dependency code and shaves
6 MB off the geth binary size.

metrics/metrics.go

@@ -27,7 +27,7 @@ var Enabled = false
 var EnabledExpensive = false
 
 // enablerFlags is the CLI flag names to use to enable metrics collections.
-var enablerFlags = []string{"metrics", "dashboard"}
+var enablerFlags = []string{"metrics"}
 
 // expensiveEnablerFlags is the CLI flag names to use to enable metrics collections.
 var expensiveEnablerFlags = []string{"metrics.expensive"}