MAINT: Remove unnecessary `graph_utils.py` functions

[nawtrey]

• Removes add_node_attribute and add_graph_attribute from graph_utils.py

• Removes the associated test, test_add_attributes, from test_kda.py

[nawtrey]

These functions are being removed because they don't add any functionality really, they just change the API for assigning attributes.

Tests are passing, docs look good, codecov is unchanged. Merging.