Made three projects packable

The projects made package are:
- Robust.Client
- Robust.Shared
- Robust.Shared.Maths
The author that made it not so is no longer here.
Assuming they did it out of a lack of need, this is now enabled.
The output path of the nupkg is noncomformant, but making it conform may
risk far more concerning breaking changes or regressions.

Author: Notarin

Robust.Client/Robust.Client.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="..\MSBuild\Robust.Engine.props" />
   <PropertyGroup>
-    <IsPackable>false</IsPackable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <OutputType>WinExe</OutputType>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>

Robust.Shared.Maths/Robust.Shared.Maths.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="..\MSBuild\Robust.Engine.props" />
   <PropertyGroup>
-    <IsPackable>false</IsPackable>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>

Robust.Shared/Robust.Shared.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="..\MSBuild\Robust.Engine.props" />
   <PropertyGroup>
-    <IsPackable>false</IsPackable>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <NoWarn>CA1416</NoWarn>