Remove "flattened" apexes

Now soong doesn't build "flattened" apexes.

Bug: 279835185
Test: m nothing (soong tests)
Test: m && launch_cvd (cuttlefish)
Change-Id: Id3c540ece1a15cecacc185da9aa17285edd2f493

android/config.go

@@ -1645,10 +1645,6 @@ func (c *config) AmlAbis() bool {
 	return Bool(c.productVariables.Aml_abis)
 }
 
-func (c *config) FlattenApex() bool {
-	return Bool(c.productVariables.Flatten_apex)
-}
-
 func (c *config) ForceApexSymlinkOptimization() bool {
 	return Bool(c.productVariables.ForceApexSymlinkOptimization)
 }