test: drop bogus check for YAJL from libxl test/mock

test: drop bogus check for YAJL from libxl test/mock

The libxlmock.c conditionalizes on WITH_YAJL, but this mock is
used from other tests which only conditionalize on WITH_LIBXL.
The libxl code does not have any dependancy on YAJL, so the
bogus condition can be removed from the mock and also from
libxlxml2domconfigtest.c

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>