Get rid of INSTALL.MacOSX?

[Jean-Marc Lasgouttes]

The last remaining bit of TODO.killqt4 is INSTALL.MacOSX. I thought we 
wanted to update it, but I fail to see what should be kept in this file 
as it is.

Stephan, do we have a reason to keep it?

Later, you could create a new INSTALL.macOS with up-to-date information 
if you feel that the file is needed.

Thoughts?

JMarc