Remove InstallMessageProc flag from OS X interactors.

This flag has never been used by these interactors, it is only there
as the result of a lazy copy-and-paste when these classes were created
using the vtkWin32RenderWindowInteractor as boilerplate code.

Change-Id: I29893d60697302198b1429f69e2af77f53881e01
Merge topic 'consolidate-interactors' into master

abad5e11 Remove InstallMessageProc flag from OS X interactors.