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

rust/rust.go

@@ -341,10 +341,6 @@ func (mod *Module) Bootstrap() bool {
 	return Bool(mod.Properties.Bootstrap)
 }
 
-func (mod *Module) MustUseVendorVariant() bool {
-	return true
-}
-
 func (mod *Module) SubName() string {
 	return mod.Properties.SubName
 }