drm/amd/display: Remove unused link_enc_cfg_get_link_enc_used_by_stream

link_enc_cfg_get_link_enc_used_by_stream() is no longer used after
2021's:
commit 6366b003 ("drm/amd/display: Maintain consistent mode of
operation during encoder assignment")
which introduces and uses the _current version instead.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>