[smuxi] Common: fixed build on .NET 3.5 profile

[Mirco M. M. Bauer <mmmbauer src gnome org>]

commit 6de14c934d172567ece9c606f3581b51ab235ba0
Author: Mirco Bauer <meebey meebey net>
Date:   Sun Jan 25 04:06:10 2015 +0100

    Common: fixed build on .NET 3.5 profile

 src/Common/SingleApplicationInstance.cs |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)
---
diff --git a/src/Common/SingleApplicationInstance.cs b/src/Common/SingleApplicationInstance.cs
index 9743f23..935452e 100644
--- a/src/Common/SingleApplicationInstance.cs
+++ b/src/Common/SingleApplicationInstance.cs
@@ -254,7 +254,6 @@ namespace Smuxi.Common
                     mutex.ReleaseMutex();
                 }
                 mutex.Close();
-                mutex.Dispose();
             }
 
 #if MONO_UNIX