Remove deprecated functionality for ensmallen 3.x release.

[rcurtin]

This removes the ParetoFront() and ParetoSet() functions from the multi-objective optimizers, since we are about to do a major version bump of ensmallen. There are also a couple other deprecated things I found and removed.

[conradsnicta]

@rcurtin Recommend making a note in HISTORY.md specifying what was removed and what functionality should be used instead. Many people tend to compile stuff while disregarding the deprecation warning messages. Only when something breaks they get interested and start looking at what happened. Having an explanation in HISTORY.md would help them fix their code.

[coatless]

@rcurtin to follow-up on @conradsnicta's point; maybe include a code snippet of moving from v2.y.z to v3.y.z line showing old and, then, new code instead of the documentation redirect. Esp if v3.y.z has coot support. I think that probably will need a concise here's GPUs and, then, a longer version.

[rcurtin]

Thanks, agreed. I just pushed a better message with a more coherent description of how to convert code. It's a little more tedious to write all that out, but better than unhappy users who get frustrated and find a different library. (Although to my knowledge there are simply zero other choices in C++ for something like what ensmallen does!)