Remove the private function addActionToEvent()

The function is no longer necessary now that we have addAction() as
the sole public method for adding actions to events.  Previously we
needed this function to help with addActionWithInterval() but now
that the method is no longer part of the API we simply the code base
by getting rid of this internal function.

src/Luvent.lua

@@ -175,23 +175,6 @@ local function findAction(event, actionToFind)
     return false, nil
 end
 
---- Add a new action to an event.
---
--- This function is private to Luvent and exists to factor out the
--- common logic in the public API for adding actions to events.
---
--- @see Luvent:addAction
-local function addActionToEvent(event, action)
-    assert(isActionCallable(action) == true)
-
-    -- We do not allow adding an action more than once to an event.
-    if event:callsAction(action) then return end
-
-    local new = newAction(action)
-    table.insert(event.actions, new)
-    return new.id
-end
-
 --- Add an action to an event.
 --
 -- It is not possible to add the same action more than once.
@@ -203,7 +186,14 @@ end
 --
 -- @see isActionCallable
 function Luvent:addAction(actionToAdd)
-    return addActionToEvent(self, actionToAdd)
+    assert(isActionCallable(actionToAdd) == true)
+
+    -- We do not allow adding an action more than once to an event.
+    if self:callsAction(actionToAdd) then return end
+
+    local new = newAction(actionToAdd)
+    table.insert(self.actions, new)
+    return new.id
 end
 
 --- Remove an action from an event.