Don't set JAVA_HOME on Mac OS X

...for one, it doesn't appear necessary there ("needed for awt"), and for
another setting it according to Oracle's JRE 7 would prevent a subsequent
visit to the Advanced preferences pane from listing Apple's JRE 6, as launching
that java and reading its system properties would be fooled by the JAVA_HOME and
report the Oracle system properties instead.

Change-Id: I02d82de6113b44b3cae3fb55cd83177fe852d769