[installd] Remove boot complete marker

ART doesn't load the boot image from /data anymore and the marker is
useless.

Test: device boots
Bug: 143899228

(cherry picked from commit 9308a544)

Merged-In: I78defa45b52b868da9861a3aac28d15d8c453d16
Change-Id: I1d06767e29c76d2f19d35bd8ae53bc5d27bda810