gstreamer: remove --disable-registry from configure options

--disable-registry has no dependency on libxml2 and should not be
linked to BR2_PACKAGE_GSTREAMER_LIBXML2. If we want to disable
registry then we should add another option.

Signed-off-by: Lionel Landwerlin <llandwerlin@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>