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

BUILDING.md

@@ -34,14 +34,7 @@ $ ./configure
 $ make
 ```
 
-If your Python binary is in a non-standard location or has a
-non-standard name, run the following instead:
-
-```console
-$ export PYTHON=/path/to/python
-$ $PYTHON ./configure
-$ make
-```
+Note that the above requires that `python` resolve to Python 2.6 or 2.7 and not a newer version.
 
 To run the tests: