rm igraph::estimate_ functions that will soon be deprecated

[maelle]

Cf igraph/rigraph#844

[szhorvat]

Just noticed something small which @thomasp85 might want to fix at the same time:

centrality_closeness() has cutoff=NULL as default while centrality_betweenness() has cutoff=-1. The meaning is identical, but for consistency, it would be nice to use NULL for both.

@thomasp85, I also wanted to let you know that igraph has an implementation of harmonic centrality with cutoffs. You might consider using this for centrality_closeness_harmonic().

[thomasp85]

Thanks, Maelle!