Only on macOS, the SDK used to expect javac etc. in a Commands sub-dir (which
Apple's JDK 1.6.0 has but Oracle's JDK 1.8.x don't).  However, at least both
Apple's latest JDK 1.6.0 (as available via <https://support.apple.com/kb/DL1572>
"Download Java for OS X 2015-001") and any recent Oracle JDK 1.8.x (like
jdk1.8.0_121.jdk) have a Home sub-dir that contains a "standard" sub-tree with
bin sub-dir etc., like on other platforms.  So consistently make the SDK use
that instead.

This removes the JAVABIN Make variable from settings.mk.  It is assumed to not
be used by client code.

Change-Id: Ie0ad647f489528444dfd399c2f00500b772d3288