Create image apexes by default instead of flattened apexes

This is a list of all the internal master products that
use flattened apexes before and after this commit:
https://diff.googleplex.com/#key=AcyUcqOwqpXZ

Apexes can still be flattened by setting `TARGET_FLATTEN_APEX := true`
or `OVERRIDE_TARGET_FLATTEN_APEX := true` in their product config.

Bug: 254205429
Test: presubmits
Change-Id: I5b15f0fc9e149730fd33722632a8ac14432ab757
diff --git a/core/board_config.mk b/core/board_config.mk
index af05973..b1b7b81 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -996,19 +996,13 @@
 endif
 
 ###########################################
-# APEXes are by default flattened, i.e. non-updatable, if not building unbundled
-# apps. It can be unflattened (and updatable) by inheriting from
-# updatable_apex.mk
+# APEXes are by default not flattened, i.e. updatable.
 #
 # APEX flattening can also be forcibly enabled (resp. disabled) by
 # setting OVERRIDE_TARGET_FLATTEN_APEX to true (resp. false), e.g. by
 # setting the OVERRIDE_TARGET_FLATTEN_APEX environment variable.
 ifdef OVERRIDE_TARGET_FLATTEN_APEX
   TARGET_FLATTEN_APEX := $(OVERRIDE_TARGET_FLATTEN_APEX)
-else
-  ifeq (,$(TARGET_BUILD_APPS)$(TARGET_FLATTEN_APEX))
-    TARGET_FLATTEN_APEX := true
-  endif
 endif
 
 ifeq (,$(TARGET_BUILD_UNBUNDLED))