[art] 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 e6f21c4597de7c4060ef5ad30d07ce9ef16d01ad)

Merged-In: I3b60d24c87a8f88cf8fc4a032ebfbf55db74a944
Change-Id: I999ef31b2edf6d9e4e7a03662ed7431e778c19b0