Remove libxml2-schemas as aosp's libxml2 handles it now

Since bd7379f5dc691a177b8e3bf3b765e8388bf06b4c in libxml2, Android's history
have been reconciled with libxml2 vanilla's history and a lot of features
were activated including the Schema validation.

The libxml2-schema hacked target is now useless.

Signed-off-by: David Wagner <david.wagner@intel.com>