doc: nvdimm: remove reference to non-existent CONFIG_NFIT_TEST

The test driver is in tools/testing/nvdimm and cannot be selected by a
config option.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Link: https://lore.kernel.org/r/20200415211654.10827-1-msuchanek@suse.de


Signed-off-by: Jonathan Corbet <corbet@lwn.net>