boot_fit.mk: drop the build of boot_fit.img am: 1a12e4f98f am: 150159625d am: cce121eead

Original change: https://android-review.googlesource.com/c/device/ti/beagle-x15/+/2005355

Change-Id: I55c7f5a1bb657c9615f39605339ea23426466b7b