[!] deprecate realtime metrics (#925)

Apparently functionality was broken for several years but nobody noticed
it.

- Delete `stat_activity_realtime` and `show_plans_realtime` metrics
- Remove realtime template table and partitioning logic
- Clean up Go code, tests, and SQL functions
- Delete realtime Grafana dashboards
- Update documentation to remove realtime references

Author: pashagolub

docs/gallery/dashboards.md

@@ -41,12 +41,6 @@ Replication Lag
 Server Log Events
 ![Server Log Events](server_log_events.png){data-gallery="dashboards"}
 
-Realtime Execution Plans
-![Realtime Execution Plans](show_plans_realtime.png){data-gallery="dashboards"}
-
-Stat Activity Realtime
-![Stat Activity Realtime](stat_activity_realtime.png){data-gallery="dashboards"}
-
 Stat Statements SQL Search
 ![Stat Statements SQL Search](stat_statements_sql_search.png){data-gallery="dashboards"}
 
@@ -60,4 +54,4 @@ System Statistics
 ![System Statistics](system_stats_psutil.png){data-gallery="dashboards"}
 
 Tables Top
-![Tables Top](tables_top.png){data-gallery="dashboards"}
+![Tables Top](tables_top.png){data-gallery="dashboards"}