Drop REASON_CORE_APP.

After ensuring apps used by system server are speed compiled
on /system, we don't need a special reason anymore.

bug:36532541
bug:33799337

Test: m -j32 && boot
Change-Id: I1fb894f0f5b796cb9d59ed01a4afb2709c0f5496
(cherry picked from commit b70e4bfe)