Set NAPARI_CONFIG to clean settings (#310)

# Description
During work on 0.4.19 I had problem with build docs because of local
settings modification.

This PR set the `NAPARI_CONFIG` variable to an empty value to prevent
usage of local settings during build docs.

Makefile

@@ -27,10 +27,10 @@ prep-docs:
 	python $(docs_dir)/_scripts/prep_docs.py
 
 docs-build: prep-docs
-	NAPARI_APPLICATION_IPY_INTERACTIVE=0 sphinx-build -b html docs/ docs/_build -D sphinx_gallery_conf.examples_dirs=$(GALLERY_PATH) $(SPHINXOPTS)
+	NAPARI_CONFIG="" NAPARI_APPLICATION_IPY_INTERACTIVE=0 sphinx-build -b html docs/ docs/_build -D sphinx_gallery_conf.examples_dirs=$(GALLERY_PATH) $(SPHINXOPTS)
 
 docs-xvfb: prep-docs
-	NAPARI_APPLICATION_IPY_INTERACTIVE=0 xvfb-run --auto-servernum sphinx-build -b html docs/ docs/_build -D sphinx_gallery_conf.examples_dirs=$(GALLERY_PATH) $(SPHINXOPTS)
+	NAPARI_CONFIG="" NAPARI_APPLICATION_IPY_INTERACTIVE=0 xvfb-run --auto-servernum sphinx-build -b html docs/ docs/_build -D sphinx_gallery_conf.examples_dirs=$(GALLERY_PATH) $(SPHINXOPTS)
 
 docs: clean docs-install docs-build