Remove mainline boot.img from GSI targets

Originally, the mainline kernel is for Android12.
The mainline kernel is changing to the next Android version.
We don't need to includes mainline boot.img in GSI targets.

Bug: 191120961
Test: TH and check the built result
Change-Id: Icd7677f362077eb675760d51a8f01c5db8baf7e1