I was getting the same warning i did this  :

1. unload project
2. edit project properties i.e .csproj 3> add follow tag

4)Reload project 5) Done...

<PropertyGroup>
<ResolveAssemblyWarnOrErrorOnTargetArchitectureMismatch>
None
</ResolveAssemblyWarnOrErrorOnTargetArchitectureMismatch>
</PropertyGroup>

I was getting the same warning i did this:

1. unload project

2. edit project properties i.e .csproj

3. add the following tag:

    <PropertyGroup>
        <ResolveAssemblyWarnOrErrorOnTargetArchitectureMismatch>
            None
        </ResolveAssemblyWarnOrErrorOnTargetArchitectureMismatch>
    </PropertyGroup>
   

4. Reload the project