Removed unneeded reference.

Author: David Britch

src/Mobile/eShopOnContainers/eShopOnContainers.Droid/eShopOnContainers.Droid.csproj

@@ -161,7 +161,6 @@
       <HintPath>..\..\..\..\packages\Microsoft.Net.Http.2.2.29\lib\monoandroid\System.Net.Http.Primitives.dll</HintPath>
       <Private>True</Private>
     </Reference>
-    <Reference Include="System.ObjectModel" />
     <Reference Include="System.Xml.Linq" />
     <Reference Include="System.Xml" />
     <Reference Include="Validation, Version=2.2.0.0, Culture=neutral, PublicKeyToken=2fc06f0d701809a7, processorArchitecture=MSIL">