Remove `stringify_path` function in SLI-free NEST

[nicolossus]

#2920 introduced the stringify_path function in order to support os.PathLike objects, such as pathlib.Path, to be passed as filenames:

nest-simulator/pynest/nest/lib/hl_api_helper.py

Lines 151 to 173 in 0d9715d

	def stringify_path(filepath):
	"""
	Convert path-like object to string form.
	Attempt to convert path-like object to a string by coercing objects
	supporting the fspath protocol to its ``__fspath__`` method. Anything that
	is not path-like, which includes bytes and strings, is passed through
	unchanged.
	Parameters
	----------
	filepath : object
	Object representing file system path.
	Returns
	-------
	filepath : str
	Stringified filepath.
	"""
	if isinstance(filepath, os.PathLike):
	filepath = filepath.__fspath__() # should return str or bytes object
	return filepath

This function is only necessary because of the SLI layer between PyNEST and the NEST kernel, and should be removed once NEST is free of SLI.