Revert the feature added by commit v3.1.0-rc1~420^2~2 (FindOpenGL:
Provide imported targets for GL and GLU, 2014-05-31).  Unfortunately it
does not work on Windows because the full path to each library file is
not actually known.  The IMPORTED_LOCATION of an imported target must be
a full path, but OPENGL_gl_LIBRARY is just 'opengl32' on Windows because
the actual library file is in some implicit link directory that we may
know know.

More infrastructure will be needed in CMake to allow a name-only
imported library.  Until that exists, we will not be able to provide
imported targets in FindOpenGL.