drm/etnaviv: Remove empty etnaviv_gem_prime_vunmap()

The function etnaviv_gem_prime_vunmap() is empty. Remove it before
changing the interface to use struct drm_buf_map.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201103093015.1063-4-tzimmermann@suse.de