Removed old r3 plug-ins from the DFHack directory. GH #2

Needed for r4 (and removed by this commit):

* stockflow.plug.(dll|so)
* stonesense.plug.dll
* trueTransformation.plug.dll

I'm most concerned about trueTransformation.plug.dll going; I
haven't looked into what it does, but it sure sounds essential.

Harmless removals:

* diggingInvaders.plug.so (diggingInvaders.plug.dll is still there for r4)