revert: remove system monitor panel toggle and event handling feature

This reverts commit aee7030b30, undoing the
addition of the system monitor feature including its panel toggle UI and
associated event handling logic.

include/dwn.h

@@ -107,7 +107,6 @@ typedef struct Workspace Workspace;
 typedef struct Monitor Monitor;
 typedef struct Panel Panel;
 typedef struct Config Config;
-typedef struct SystemMonitor SystemMonitor;
 
 struct Client {
     Window window;
@@ -177,8 +176,6 @@ typedef struct {
     Panel *top_panel;
     Panel *bottom_panel;
 
-    SystemMonitor *sysmon;
-
     Config *config;
 
     bool running;