Replace get_allowable_sampledata_headers with get_case_attributes
Replace get_allowable_sampledata_headers with get_case_attributes

* wqflask/wqflask/metadata_edits.py: Remove
"get_allowable_sampledata_headers" import.
(display_phenotype_metadata): Use
"get_case_attributes" to fetch all the allowable
case attributes.

genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2