Phase out all _a targets

The _a targets are build for Legacy GSI for non-system-as-root
partition layout devices. The Android build system is not able
to build a system image with non-system-as-root anymore.
Also, `init` supports mounting system-as-root system image at
all, all devices should no problem to mount it.

Bug: 124278972
Test: none
Change-Id: I9c56e7e5d8517a630854f030616c3b3ecb94b9a8