Revert create_project_ssh_key error work-arounds

[displague]

It should now be safe to remove the workaround to #78:

packet-python/packet/Manager.py

Lines 222 to 232 in 927684b

	except ResponseError as e:
	if e.response.status_code == 422:
	# Try to pluck the SSH key from the listing API
	keys = [
	key
	for key in self.list_ssh_keys()
	if key.key.strip() == public_key.strip()
	]
	if len(keys) == 1:
	return keys.pop()
	raise

The function should be returned to its original composition from #80.

Other changes from #80 can be retained.