requirements: no longer drop generaldelta requirement with revlogv2
A repository could use a mix of revlogv1 and revlogv2, making the requirements
still necessary. Overall we should move away from the "requirements" file being
used a way to configure the repository and stick to it "what do you need to
access this repository". However this is a wider work for another time.
In addition the logic we just dropped was confusing the hg debugformat
command, breaking the upgrade code and inconsistent (eg: sparse-revlog is also
implied by revlogv2).
Finally, multiple other config option would imply the use of the revlogv2
requirements, without drop the generaldelta one, leading to more
Differential Revision: https://phab.mercurial-scm.org/D10612