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