Remove unnecessary symlink for non-flattend APEXes

This change fixes a bug that symlinks to the system partition are
created in /system/apex/<apex_name> directories even when the APEXes are
non-flattened. The symlinks are needed only for flattened APEX (of
course regardless of whether the APEX is a primary one or not).

Bug: N/A
Test: examine /system/apex directory manually
Change-Id: I00bb1423d0a2497408f05e49767b42437210bab8