Cleanup use case part in doc repo

The use cases are hosted in integration repositories
There were 2 dirctories referencing use cases in doc repo
- guides/onap-developer/use-cases
- use-cases

they both dealt with vfw and were not up to date
the vfw call flows are also in integration repository

no update since beijing

Issue-ID: INT-1776

Signed-off-by: mrichomme <morgan.richomme@orange.com>
Change-Id: I61b8eb80381d13f45bfdd4a7639ff5364d02186b