Remove accidental System.Core.dll from CascadiaPackage (#9153)

A bug in VS 16.8 makes the WAP packaging project copy System.Core.dll
from the CLR into all WAP packages. We don't need it, and it adds 300kb
to our package (670kb uncompressed).

VS 16.9 sets the AddAdditionalExplicitAssemblyReferences to suppress
this assembly. If we do the same, we can avoid the reference *and* be
eady for VS 16.9.

src/cascadia/CascadiaPackage/CascadiaPackage.wapproj

@@ -15,6 +15,10 @@
     <AppxOSMaxVersionTestedReplaceManifestVersion>false</AppxOSMaxVersionTestedReplaceManifestVersion>
     <OCExecutionAliasName Condition="'$(WindowsTerminalBranding)'==''">wtd</OCExecutionAliasName>
     <OCExecutionAliasName Condition="'$(OCExecutionAliasName)'==''">wt</OCExecutionAliasName>
+    <!-- VS 16.8 causes WAP projects to accidentally package System.Core.dll (from the CLR).
+         This has been fixed in VS 16.9 using the property below. It's safe for us to include
+         it here, even after 16.9 comes out. -->
+    <AddAdditionalExplicitAssemblyReferences>false</AddAdditionalExplicitAssemblyReferences>
   </PropertyGroup>
   <PropertyGroup>
     <ProjectGuid>CA5CAD1A-224A-4171-B13A-F16E576FDD12</ProjectGuid>