sch: event funcs must be implemented

leftover from original impl attempt

Author: mcspr

code/espurna/scheduler.cpp

@@ -1557,14 +1557,8 @@ struct Source {
     virtual ~Source();
 
     virtual const datetime::Minutes& minutes() const = 0;
-
-    virtual bool before(const datetime::Context&) {
-        return true;
-    }
-
-    virtual bool after(const datetime::Context&) {
-        return true;
-    }
+    virtual bool before(const datetime::Context&) = 0;
+    virtual bool after(const datetime::Context&) = 0;
 };
 
 constexpr datetime::Minutes Source::DefaultMinutes;