Retire pgrouting-contrib repository

[dkastl]

I would suggest to retire pgrouting-contrib repository when we release pgRouting v3.0 ... except someone has good arguments to keep and use it.

https://github.com/pgRouting/pgrouting-contrib

The original idea was to provide additional functions, that were long time ago added to the main repository "accidentally" but were too much extra work to maintain.

But since then there is very little or actually no activity in the repository. It doesn't seem to be of any value for me.