Remove duplicated package reference at GxMail.csproj (#662)

(cherry picked from commit 3c4309c)

Author: claudiamurialdo

dotnet/src/dotnetcore/GxMail/GxMail.csproj

@@ -72,7 +72,6 @@
     <PackageReference Include="MimeKit" Version="3.1.1" />
     <PackageReference Include="Microsoft.Identity.Client" Version="4.42.0" />
     <PackageReference Include="OpenPop" Version="2.0.6.2" />
-		<PackageReference Include="MailKit" Version="3.1.1" />
 	<PackageReference Include="Org.Mentalis.Security" Version="1.0.0" />
   </ItemGroup>