Removed `bsSize` and `bsStyle` modifier mixins

Please ignore this PR if this is miss guided but i cant figure out what these mixins would be useful for. They are not used anywhere within this lib's components, plus i can't see how they would be useful for external components since they just define `bsSize` and `bsStyle` props which do nothing on their own.

Also this change removes a large number of files which is a big plus :)

Is there anything i'm missing here?