Remove ImplicitUsings label, not supported by Net 6.0

Author: oscarsj

csharp/ql/integration-tests/all-platforms/autobuild/Files.expected

@@ -1,3 +1,3 @@
 | Program.cs:0:0:0:0 | Program.cs |
-| obj/Debug/net9.0/.NETCoreApp,Version=v5.0.AssemblyAttributes.cs:0:0:0:0 | obj/Debug/net9.0/.NETCoreApp,Version=v5.0.AssemblyAttributes.cs |
-| obj/Debug/net9.0/autobuild.AssemblyInfo.cs:0:0:0:0 | obj/Debug/net9.0/autobuild.AssemblyInfo.cs |
+| obj/Debug/net5.0/.NETCoreApp,Version=v5.0.AssemblyAttributes.cs:0:0:0:0 | obj/Debug/net5.0/.NETCoreApp,Version=v5.0.AssemblyAttributes.cs |
+| obj/Debug/net5.0/autobuild.AssemblyInfo.cs:0:0:0:0 | obj/Debug/net5.0/autobuild.AssemblyInfo.cs |

csharp/ql/integration-tests/all-platforms/autobuild/autobuild.csproj

@@ -2,8 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net9.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
+    <TargetFramework>net5.0</TargetFramework>
     <Nullable>enable</Nullable>
   </PropertyGroup>