Audio: Remove the hack to buid a libxml2 with schemas

BZ: 218524

The "Schemas" feature is enabled in Android-L,
so there is no need for this out-of-tree compilation
anymore.

So this patch deletes its compilation in
parameter-framework project.

Change-Id: I111afa3e12fa80c2871bae475d550bc49c4440cc
Signed-off-by: pafonsoX <philippex.afonso@intel.com>
Signed-off-by: David Wagner <david.wagner@intel.com>