Do not pass TARGET_FLATTEN_APEX to soong

Soong no longer supports "flattened" apex. This makes
TARGET_FLATTEN_APEX no-op.

Will remove TARGET_FLATTEN_APEX later.

Bug: 279835185
Test: m && launch_cvd (cuttlefish)
Change-Id: I27c6181f5107fd27552304c5dd25d2c0a21c4bec
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 5f62ec6..82d7932 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -226,7 +226,6 @@
 $(call add_json_str,  TotSepolicyVersion,                $(TOT_SEPOLICY_VERSION))
 $(call add_json_list, PlatformSepolicyCompatVersions,    $(PLATFORM_SEPOLICY_COMPAT_VERSIONS))
 
-$(call add_json_bool, Flatten_apex,                      $(filter true,$(TARGET_FLATTEN_APEX)))
 $(call add_json_bool, ForceApexSymlinkOptimization,      $(filter true,$(TARGET_FORCE_APEX_SYMLINK_OPTIMIZATION)))
 
 $(call add_json_str,  DexpreoptGlobalConfig,             $(DEX_PREOPT_CONFIG))