cros_vm_constants: remove obsolete constants

No CS hits for VMDK, VMX, VBOX constants. Last use of DEFAULT_MEM has
also just been removed now that image_to_vm.sh recommends using cros_vm.

BUG=chromium:782664
TEST=Looked up CS, Pre-CQ and CQ should confirm.

Change-Id: Iaedd835c99db0033b6b72e606aa1225531fdd353
Reviewed-on: https://chromium-review.googlesource.com/889409
Commit-Ready: Nicolas Norvez <norvez@chromium.org>
Tested-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
diff --git a/lib/cros_vm_constants.sh b/lib/cros_vm_constants.sh
index ab87f96..70ae125 100644
--- a/lib/cros_vm_constants.sh
+++ b/lib/cros_vm_constants.sh
@@ -5,8 +5,4 @@
 # Common constants for vm scripts.
 
 # Default values for creating VM's.
-DEFAULT_MEM="1024"
-DEFAULT_VMDK="ide.vmdk"
-DEFAULT_VMX="chromiumos.vmx"
-DEFAULT_VBOX_DISK="os.vdi"
 DEFAULT_QEMU_IMAGE="chromiumos_qemu_image.bin"