Remove unnecessary Future.delayed() from SideMenu.dispose()

[gradycsjohnson]

It seems to be unnecessary to call Future.delayed() inside the SideMenu.dispose() method, and doing so causes async tests to fail with the timer still pending error:

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following assertion was thrown running a test:
A Timer is still pending even after the widget tree was disposed.
'package:flutter_test/src/binding.dart':
Failed assertion: line 1527 pos 12: '!timersPending'

When the exception was thrown, this was the stack:
#2      AutomatedTestWidgetsFlutterBinding._verifyInvariants (package:flutter_test/src/binding.dart:1527:12)
#3      TestWidgetsFlutterBinding._runTestBody (package:flutter_test/src/binding.dart:1034:7)

Removing solves this problem without seeming to cause any issues.