drm/tee_shm: Drop dma_buf_k(unmap) support

There's no in-tree users anymore.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Cc: tee-dev@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-13-daniel.vetter@ffwll.ch