Clean up TARGET_FLATTEN_APEX lines in mainline products

Remove redundant TARGET_FLATTEN_APEX lines from mainline board configs.
Remove redundant "inherit updatable_apex.mk" lines from
mainline_system_$arch makefiles.
mainline_system.mk now always install APEX into /system partition.
Products who need flattened APEXes would have their flattened APEXes
installed into /system_ext.

Test: Build mainline_system_arm64 and validate noop
Change-Id: I9ee4af529a4bd554cb8707cfc260d6b912fd5fff