drm/nouveau/gem: Remove unused function

Remove the function domain_to_ttm() that is not used anywhere.

This was partially found by using a static code analysis program
called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>