[MultiDisplay] remove emulator_system.mk

emulator_system.mk was required by MultiDisplay app to install and
run in system/priv_app. Since MultiDisplay app has been removed to
system_ext partition, it is safe to remove the emulator_system.mk

Bug: 141870812

Test: MultiDisplay has been verified to work with sdk_phone_x86/64 user/userdebug
sdk_gphone_x86/64 user/userdebug

Change-Id: I8db33ce9c037585ba65f09164afcfb2655710b81