find_gfs_with_extra_contributors accesses keys that aren't used

[fiendish]

Describe the bug
find_gfs_with_extra_contributors accesses keys that aren't used causing errors

kf-utils-python/kf_utils/dataservice/descendants.py

Lines 42 to 46 in bc191c1

	has_extra_contributors = {
	"mixed_visibility": set(),
	"hidden": set(),
	"visible": set(),
	}

vs

kf-utils-python/kf_utils/dataservice/descendants.py

Lines 62 to 64 in bc191c1

	has_extra_contributors["all_hidden"].add(g)
	else:
	has_extra_contributors["all_visible"].add(g)

and

kf-utils-python/kf_utils/dataservice/descendants.py

Line 146 in bc191c1

descendant_kfids["genomic-files"] -= extra_contrib_gfs["all_hidden"]