Cleanup irrelevant aboot build target

Legacy .PHONY aboot build target is not relevant on FP3. Instead we have
two relevant targets/outputs:

* aboot.img: Copied signed bootloader from prebuilts ("radio" images)
* `m out/target/product/FP3/emmc_appsboot.mbn`: Build **unsigned**
  bootloader from source.

Change-Id: I06286242d470dae5297d2e8c62838c03905b38bf
1 file changed