remove explicit file check so IRIs can be parsed

[srmnitc]

This pull request includes a change to the tools4rdf/network/parser.py file to improve the robustness of the parse_ontology function by removing an unnecessary file existence check.

Codebase simplification:

• tools4rdf/network/parser.py: Removed the check for file existence (os.path.exists(infile)) and the corresponding FileNotFoundError exception to streamline the parse_ontology function.

closes #38