remove forgotten refs to Mono.Posix.dll

prebuild.xml

@@ -497,7 +497,6 @@
       <Reference name="OpenSim.Framework.Monitoring"/>
       <Reference name="OpenSim.Framework.Servers"/>
       <Reference name="OpenSim.Framework.Servers.HttpServer"/>
-      <Reference name="Mono.Posix" path="../../../bin/"/>
       <Reference name="Nini" path="../../../bin/"/>
       <Reference name="log4net" path="../../../bin/"/>
 
@@ -1792,7 +1791,6 @@
       <Reference name="System.Core"/>
       <Reference name="System.Xml"/>
       <Reference name="Mono.Addins" path="../../../bin/"/>
-      <Reference name="Mono.Posix" path="../../../bin/"/>
       <Reference name="NDesk.Options" path="../../../bin/"/>
       <Reference name="OpenMetaverseTypes" path="../../../bin/"/>
       <Reference name="OpenMetaverse.StructuredData" path="../../../bin/"/>