Remove of scale parameter of eigen_centrality()

[szhorvat]

In C/igraph 1.0, the scale parameter of eigen_centrality() will be removed and the function will always behave as if scale=TRUE were used. Details are at igraph/igraph#2446

This issue is to start preparing for this change on the R side, probably by deprecating the scale parameter.

Note that no real functionality is lost: scale=FALSE doesn't guarantee anything about any kind of scaling/normalization, while scale=TRUE uses a specific normalization. People should never have relied on scale=FALSE, so warning those who do soon is a good thing.

CC @maelle