Remove VndkUseCoreVariant related code from build

VndkUseCoreVariant feature is deprecated along with VNDK. This change
removes related code from Soong.

Bug: 330100430
Test: AOSP CF build succeeded
Change-Id: Ie182c9e4dc9cf0a4fe9d5fddf5b36754ddb53d18

cc/linkable.go

@@ -165,7 +165,6 @@ type LinkableInterface interface {
 	// IsVndkSp returns true if this is a VNDK-SP module.
 	IsVndkSp() bool
 
-	MustUseVendorVariant() bool
 	IsVndk() bool
 	IsVndkExt() bool
 	IsVndkPrivate() bool