cmake: only set output paths with multiple outputs

This brings cmake inline with the behaviour used for configure
scripts, defined in multiple-outputs.sh.  It's important because
that setup hook will only set shareDocName if multiple outputs are in
use (and setOutputFlags hasn't been disabled).  So previously,
CMAKE_INSTALL_DOCDIR would be set to $out/share/doc for single-output
derivations, instead of $out/share/doc/$shareDocName, which would
result in collisions.

Since this hook now uses the setOutputFlags variable, I had to remove
the empty assignment of it added in
a714284d.

Fixes: https://github.com/NixOS/nixpkgs/issues/82304