eliminate_kronecker does not respect index locality

[kpeeters]

With the current tree format allowing for dummy indices inside powers to be repeated elsewhere, it is possible that a Kronecker delta contains an index which is also present inside a \pow{...}{...} node, but which obviously is not contracted with it. The eliminate_kronecker delta does not respect this, which leads to broken expressions. See https://cadabra.science/qa/2435/index-replacement-problem .