doc: remove failing workaround in BUILDING.md

Updated BUILDING.md, removing workaround for Python conflicts that
didn't work.

PR-URL: https://github.com/nodejs/node/pull/8763


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Refs: https://github.com/nodejs/node/pull/8763