makefile: stop setting unused HGEXTDIR variable during osx build

The variable was added in a38ed42cd23c (osx: include chg by default,
2017-03-20), but I can't find any others references to the variable in
that commit or in any other commits.

Differential Revision: https://phab.mercurial-scm.org/D8915

Makefile

@@ -234,7 +234,6 b' osx:'
 	make -C contrib/chg \
 	  HGPATH=/usr/local/bin/hg \
 	  PYTHON=/usr/bin/python2.7 \
-	  HGEXTDIR=/Library/Python/2.7/site-packages/hgext \
 	  DESTDIR=../../build/mercurial \
 	  PREFIX=/usr/local \
 	  clean install