musicbrainz: handle rebuilds

musicbrainz: handle rebuilds

When rebuilding build-native already exists.  Sadly handling the directory
existing causes cmake to forkbomb when rebuilding, so delete an existing
build-native.

Also remove a variable that cmake warns isn't being used, and just build the
tool that we need natively, not the entire library.

(From OE-Core rev: 96649fff6de8a44ff3462df14fb8e152b6caf4f1)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>