MainActivity: inline nested function for dynamic menu

Make the UI more flexible and
slightly easier to understand.

app/src/main/java/it/danieleverducci/lunatracker/MainActivity.kt

@@ -480,9 +480,6 @@ class MainActivity : AppCompatActivity() {
 
         val pickedDateTimeBegin = dateTimePicker(event.time, datePickerBegin) { pickedTime: Long ->
             sleepStart = pickedTime
-            if (datePickerEnd.visibility == View.GONE) {
-                sleepEnd = pickedTime
-            }
             updateFields()
         }