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
diff --git a/generate_extra_images.mk b/generate_extra_images.mk
index e5f23e4..ec448b8 100644
--- a/generate_extra_images.mk
+++ b/generate_extra_images.mk
@@ -215,13 +215,6 @@
 
 ###################################################################################################
 
-.PHONY: aboot
-ifeq ($(USESECIMAGETOOL), true)
-aboot: $(TARGET_SIGNED_BOOTLOADER) gensecimage_install
-else
-aboot: $(INSTALLED_BOOTLOADER_MODULE)
-endif
-
 .PHONY: kernel
 kernel: $(INSTALLED_BOOTIMAGE_TARGET) $(INSTALLED_DTBOIMAGE_TARGET)