No longer need bootable/diskinstaller.

This was only used by the vbox target, which is now dead.

default.xml

@@ -15,7 +15,6 @@
   <project path="art" name="platform/art" />
   <project path="bionic" name="platform/bionic" groups="pdk" />
   <project path="bootable/bootloader/legacy" name="platform/bootable/bootloader/legacy" />
-  <project path="bootable/diskinstaller" name="platform/bootable/diskinstaller" />
   <project path="bootable/recovery" name="platform/bootable/recovery" groups="pdk" />
   <project path="cts" name="platform/cts" groups="cts" />
   <project path="dalvik" name="platform/dalvik" />