Bug#980992: cairo: drop unused Build-Depends

[Simon McVittie]

Thanks, applied in git.

On Mon, 25 Jan 2021 at 06:50:06 +0100, Helmut Grohne wrote:
> libxt-dev

cairo 1.2.4 NEWS says "Fix xlib detection to prefer pkg-config to avoid
false libXt dependency" so I think we can be confident that this one
is dead.

> libsm-dev
> xutils-dev

I think these were also required by AC_PATH_XTRA before switching to
preferring pkg-config as the way to to find Xlib.

    smcv