Removes the differences doc

[andyscott]

It seems folks are still working on these rules. With that in mind, I'd like to remove this differences doc, as the document feels kind of petty and I'd rather these rules just exist on their own right (or get merged with bazelbuild/rules_scala).

[pauldraper]

I'd rather these rules just exist on their own right

Yet there is a compat effort to follow a bazelbuild/rules_scala interface.

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While I think some of the things need to be dropped or reworded from the doc, I believe it is helpful to compare it to other solutions, especially the semi-official bazelbuild project.

As a counterexample, there are like 4 different rules_javascript projects and it can be quite difficult to tell which ones align with my needs because there is little help in telling them apart.

I would like to help other users/contributors to understand some of the more significant differences.

I contribute to this separate project only because I believe there are important features that bazelbuild/rules_scala cannot support without a significant rewrite.

[pauldraper]

IMO not having the official "bazelbuild" blessing immediately prompts any reader to ask "why does this other thing exist." I'd like to give a brief, accurate answer.