doc: remove obsolete docs/images dir

Remove obsolete docs/images directory which contains unused
variants of the logo. The canonical location of TUF logos is
theupdateframework/artwork, which has high-resolution formats (png
and svg) for all variants of the logo.

Also see theupdateframework/artwork#3.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer