Update crash course

docs/tut/crash-course/7-cleanup.md

@@ -1,40 +0,0 @@
-# Cleanup
-
-Sometimes you may need to do some cleanup when destroying a component or after
-a side-effect from a source update. Vide provides a function `cleanup()` which
-is used to register a cleanup callback for the next time the reactive scope
-it is called in re-runs.
-
-```lua
-local vide = require(vide)
-local source = vide.source
-local cleanup = vide.cleanup
-
-local function Timer()
-    local count = source(0)
-
-    local con = game:GetService("RunService").Heartbeat:Connect(function(dt)
-        count(count() + dt)
-    end)
-
-    cleanup(function()
-        con:Disconnect()
-    end)
-
-    return create "TextButton" {
-        Position = UDim2.fromOffset(300, 300),
-        Size = UDim2.fromOffset(200, 50),
-
-        Text = function()
-            return "seconds: " .. count()
-        end,
-    }
-end
-
-mount(function() return create "ScreenGui" { Timer {} } end, game.StarterGui)
-```
-
-In the above example, this allows us to disconnect the heartbeat connection
-when the timer component is destroyed, whether that is from unmounting the app
-or if it is dynamically created by a control-flow function, which will be
-covered next.