Remove platform/ndk.

None of this is used in the platform build. The platform pieces are in
prebuilts/ndk.
Test: make checkbuild
Bug: http://b/25482763

default.xml

@@ -397,7 +397,6 @@
   <project path="kernel/tests" name="kernel/tests" />
   <project path="libcore" name="platform/libcore" groups="pdk" />
   <project path="libnativehelper" name="platform/libnativehelper" groups="pdk" />
-  <project path="ndk" name="platform/ndk" groups="generic_fs" />
   <project path="packages/apps/BasicSmsReceiver" name="platform/packages/apps/BasicSmsReceiver" groups="pdk-cw-fs,pdk-fs" />
   <project path="packages/apps/Bluetooth" name="platform/packages/apps/Bluetooth" groups="pdk-cw-fs,pdk-fs" />
   <project path="packages/apps/Browser2" name="platform/packages/apps/Browser2" groups="pdk-fs" />