remove prebuilt ramdisk

This prebuilt will no longer be maintained. For instructions on building
a boot.img with the kernel build environment please see

https://source.android.com/setup/build/building-kernels#id-bootimage

Bug: 161905980
Change-Id: Idbad3af2c09cefa292beb4376fc608f5ff359348
diff --git a/arm64/gki-ramdisk.cpio b/arm64/gki-ramdisk.cpio
deleted file mode 100644
index a3aff79..0000000
--- a/arm64/gki-ramdisk.cpio
+++ /dev/null
Binary files differ