Remove mainline_arm64.mk

mainline_arm64 is broken for quite some time. Let's just remove it
rather than trying to fix it.
IIUC this was a testing ground for mainline_system.mk when it was first
introduced. Now we have plenty of products that use mainline_system.mk
(e.g. mainline_system_arm64, aosp_arm64, aosp_<pixel>, ...).

Bug: 141443017
Test: TH
Change-Id: I8e0825b2d94134fd8e9e3edc5e2aa15dbcdf10ba
Merged-In: I08990b70d45c0663a9700f41b670513674bf7e97