Remove MRT Core .props/.targets files

[Sergio0694]

This PR removes the MRT Core .props/.targets files, as they're now in https://www.nuget.org/packages/Microsoft.Windows.SDK.BuildTools.MSIX.

[0x5bfa]

You may want to remove these lines except native ones.

WindowsAppSDK/BuildAll.ps1

Lines 323 to 330 in ad53876

	Copy-Item -Path "$MRTSourcesDirectory\packaging\MrtCore.props" -Destination "$BasePath\build"
	Copy-Item -Path "$MRTSourcesDirectory\packaging\MrtCore.PriGen.targets" -Destination "$BasePath\build"
	Copy-Item -Path "$MRTSourcesDirectory\packaging\MrtCore.References.targets" -Destination "$BasePath\build"
	Copy-Item -Path "$MRTSourcesDirectory\packaging\MrtCore.targets" -Destination "$BasePath\build"
	Copy-Item -Path "$MRTSourcesDirectory\packaging\native\MrtCore.C.props" -Destination "$BasePath\build\native"
	Copy-Item -Path "$MRTSourcesDirectory\packaging\native\MrtCore.props" -Destination "$BasePath\build\native"
	Copy-Item -Path "$MRTSourcesDirectory\packaging\native\MrtCore.targets" -Destination "$BasePath\build\native"
	Copy-Item -Path "$MRTSourcesDirectory\packaging\ProjectItemsSchema.xaml" -Destination "$BasePath\build"