(No Issue) Clean up detritus in the Makefile

We aren't pre-building wheels any more for use with pex so we no longer refer to the `wheelhouse` directory, but the script was still removing it.  This was actually surfacing on Windows/cygwin as a spurious error in some cases.

Author: adobeDan

Makefile

@@ -23,7 +23,6 @@ pex:
 		-f $(prebuilt_dir) \
 		--disable-cache \
 		--not-zip-safe .
-	-$(RM) wheelhouse
 
 test:
 	nosetests --no-byte-compile tests