Remove emulator_x86_arm board from common boards

The board will now live in the Android TV vendor partition, since the
emulator team is no longer supporting 32-bit boards.

Bug: 305610937
Test: Null
Change-Id: Ia1afb35ad15eb991bb2451c160db4f853f016587
Ignore-AOSP-First: this will be cp'ed to AOSP soon after