Remove `-d .jar` and some backend flags

[Blaisorblade]

In another post, @stuhood argued for better incremental compilation into JARs:

Asking for your feedback on sbt + Scala Center announcement

With regard to Zinc, this all sounds reasonable. The only thing that isn’t explicitly spelled out here is support for fine-grained builds. Folks using Zinc via Pants (and Bazel, presumably) are encouraged to create lots of well-defined, small modules, and to use “strict” (ie, only directly declared) dependencies. This allows for better cache hit rates against distributed build caches, but requires that Zinc support low overhead per invoke to avoid the actual compilation of ~2000 source modules…