drm/radeon/radeon_device: remove unused function

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/radeon/radeon_device.c:1961:5: warning: no previous prototype for 'radeon_debugfs_init' [-Wmissing-prototypes]
drivers/gpu/drm/radeon/radeon_device.c:1966:6: warning: no previous prototype for 'radeon_debugfs_cleanup' [-Wmissing-prototypes]

In fact, these functions are dummy, but can be removed,
so this patch removes them.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>