Remove a dummy task

Makes dotnet pack work. Kudos to @bording for figuring it out
in #700.

Author: michaelklishin

projects/client/RabbitMQ.Client/RabbitMQ.Client.csproj

@@ -1,8 +1,4 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <NuGetBuildTasksPackTargets>junk-value-to-avoid-conflicts</NuGetBuildTasksPackTargets>
-  </PropertyGroup>
-
   <PropertyGroup>
     <Description>The RabbitMQ .NET client is the official client library for C# (and, implicitly, other .NET languages)</Description>
     <VersionPrefix>6.0.0</VersionPrefix>