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/variable.go

@@ -175,10 +175,6 @@ type variableProperties struct {
 			Whole_static_libs []string `android:"arch_variant"`
 		} `android:"arch_variant"`
 
-		Flatten_apex struct {
-			Enabled *bool
-		}
-
 		Native_coverage struct {
 			Src          *string  `android:"arch_variant"`
 			Srcs         []string `android:"arch_variant"`
@@ -397,7 +393,6 @@ type productVariables struct {
 	Ndk_abis *bool `json:",omitempty"`
 
 	TrimmedApex                  *bool `json:",omitempty"`
-	Flatten_apex                 *bool `json:",omitempty"`
 	ForceApexSymlinkOptimization *bool `json:",omitempty"`
 	CompressedApex               *bool `json:",omitempty"`
 	Aml_abis                     *bool `json:",omitempty"`